I am a beginner of python and have a question, very confusing for me.
If I define a function first but within the function I have to use a variable which is defined in another function below, can I do it like this? Or how can I import the return things of another function into a function?
for example:
def hello(x,y):
good=hi(iy,ix)
"then do somethings,and use the parameter'good'."
return something
def hi(iy,ix):
"code"
return good
The scope of functions hello and hi are entirely different. They do not have any variables in common.
Note that the result of calling hi(x,y) is some object. You save that object with the name good in the function hello.
The variable named good in hello is a different variable, unrelated to the variable named good in the function hi.
They're spelled the same, but the exist in different namespaces. To prove this, change the spelling the good variable in one of the two functions, you'll see that things still work.
Edit. Follow-up: "so what should i do if i want use the result of hi function in hello function?"
Nothing unusual. Look at hello closely.
def hello(x,y):
fordf150 = hi(y,x)
"then do somethings,and use the variable 'fordf150'."
return something
def hi( ix, iy ):
"compute some value, good."
return good
Some script evaluates hello( 2, 3).
Python creates a new namespace for the evaluation of hello.
In hello, x is bound to the object 2. Binding is done position order.
In hello, y is bound to the object 3.
In hello, Python evaluates the first statement, fordf150 = hi( y, x ), y is 3, x is 2.
a. Python creates a new namespace for the evaluation of hi.
b. In hi, ix is bound to the object 3. Binding is done position order.
c. In hi, iy is bound to the object 2.
d. In hi, something happens and good is bound to some object, say 3.1415926.
e. In hi, a return is executed; identifying an object as the value for hi. In this case, the object is named by good and is the object 3.1415926.
f. The hi namespace is discarded. good, ix and iy vanish. The object (3.1415926), however, remains as the value of evaluating hi.
In hello, Python finishes the first statement, fordf150 = hi( y, x ), y is 3, x is 2. The value of hi is 3.1415926.
a. fordf150 is bound to the object created by evaluating hi, 3.1415926.
In hello, Python moves on to other statements.
At some point something is bound to an object, say, 2.718281828459045.
In hello, a return is executed; identifying an object as the value for hello. In this case, the object is named by something and is the object 2.718281828459045.
The namespace is discarded. fordf150 and something vanish, as do x and y. The object (2.718281828459045), however, remains as the value of evaluating hello.
Whatever program or script called hello gets the answer.
If you want to define a variable to the global namespace from inside a function, and thereby make it accessible by other functions in this space, you can use the global keyword. Here's some examples
varA = 5 #A normal declaration of an integer in the main "global" namespace
def funcA():
print varA #This works, because the variable was defined in the global namespace
#and functions have read access to this.
def changeA():
varA = 2 #This however, defines a variable in the function's own namespace
#Because of this, it's not accessible by other functions.
#It has also replaced the global variable, though only inside this function
def newVar():
global varB #By using the global keyword, you assign this variable to the global namespace
varB = 5
def funcB():
print varB #Making it accessible to other functions
Conclusion: variables defined in a function stays in the function's namespace. It still has access to the global namespace for reading only, unless the variable has been called with the global keyword.
The term global isn't entirely global as it may seem at first. It's practically only a link to the lowest namespace in the file you're working in. Global keywords cannot be accessed in another module.
As a mild warning, this may be considered to be less "good practice" by some.
your example program works, because the two instances of 'good' are different variables (you just happen to have both variables with the same name). The following code is exactly the same:
def hello(x,y):
good=hi(iy,ix)
"then do somethings,and use the parameter'good'."
return something
def hi(iy,ix):
"code"
return great
More details on the python scoping rules are here :
Short Description of Python Scoping Rules
The "hello" function doesn't mind you calling the "hi" function which is hasn't been defined yet, provided you don't try to actually use the "hello" function until after the both functions have been defined.
Related
Suppose I have a function like:
def foo():
x = 'hello world'
How do I get the function to return x, in such a way that I can use it as the input for another function or use the variable within the body of a program? I tried using return and then using the x variable in another function, but I get a NameError that way.
For the specific case of communicating information between methods in the same class, it is often best to store the information in self. See Passing variables between methods in Python? for details.
def foo():
x = 'hello world'
return x # return 'hello world' would do, too
foo()
print(x) # NameError - x is not defined outside the function
y = foo()
print(y) # this works
x = foo()
print(x) # this also works, and it's a completely different x than that inside
# foo()
z = bar(x) # of course, now you can use x as you want
z = bar(foo()) # but you don't have to
Effectively, there are two ways: directly and indirectly.
The direct way is to return a value from the function, as you tried, and let the calling code use that value. This is normally what you want. The natural, simple, direct, explicit way to get information back from a function is to return it. Broadly speaking, the purpose of a function is to compute a value, and return signifies "this is the value we computed; we are done here".
Directly using return
The main trick here is that return returns a value, not a variable. So return x does not enable the calling code to use x after calling the function, and does not modify any existing value that x had in the context of the call. (That's presumably why you got a NameError.)
After we use return in the function:
def example():
x = 'hello world'
return x
we need to write the calling code to use the return value:
result = example()
print(result)
The other key point here is that a call to a function is an expression, so we can use it the same way that we use, say, the result of an addition. Just as we may say result = 'hello ' + 'world', we may say result = foo(). After that, result is our own, local name for that string, and we can do whatever we want with it.
We can use the same name, x, if we want. Or we can use a different name. The calling code doesn't have to know anything about how the function is written, or what names it uses for things.1
We can use the value directly to call another function: for example, print(foo()).2 We can return the value directly: simply return 'hello world', without assigning to x. (Again: we are returning a value, not a variable.)
The function can only return once each time it is called. return terminates the function - again, we just determined the result of the calculation, so there is no reason to calculate any further. If we want to return multiple pieces of information, therefore, we will need to come up with a single object (in Python, "value" and "object" are effectively synonyms; this doesn't work out so well for some other languages.)
We can make a tuple right on the return line; or we can use a dictionary, a namedtuple (Python 2.6+), a types.simpleNamespace (Python 3.3+), a dataclass (Python 3.7+), or some other class (perhaps even one we write ourselves) to associate names with the values that are being returned; or we can accumulate values from a loop in a list; etc. etc. The possibilities are endless..
On the other hand, the function returns whether you like it or not (unless an exception is raised). If it reaches the end, it will implicitly return the special value None. You may or may not want to do it explicitly instead.
Indirect methods
Other than returning the result back to the caller directly, we can communicate it by modifying some existing object that the caller knows about. There are many ways to do that, but they're all variations on that same theme.
If you want the code to communicate information back this way, please just let it return None - don't also use the return value for something meaningful. That's how the built-in functionality works.
In order to modify that object, the called function also has to know about it, of course. That means, having a name for the object that can be looked up in a current scope. So, let's go through those in order:
Local scope: Modifying a passed-in argument
If one of our parameters is mutable, we can just mutate it, and rely on the caller to examine the change. This is usually not a great idea, because it can be hard to reason about the code. It looks like:
def called(mutable):
mutable.append('world')
def caller():
my_value = ['hello'] # a list with just that string
called(my_value)
# now it contains both strings
If the value is an instance of our own class, we could also assign to an attribute:
class Test:
def __init__(self, value):
self.value = value
def called(mutable):
mutable.value = 'world'
def caller():
test = Test('hello')
called(test)
# now test.value has changed
Assigning to an attribute does not work for built-in types, including object; and it might not work for some classes that explicitly prevent you from doing it.
Local scope: Modifying self, in a method
We already have an example of this above: setting self.value in the Test.__init__ code. This is a special case of modifying a passed-in argument; but it's part of how classes work in Python, and something we're expected to do. Normally, when we do this, the calling won't actually check for changes to self - it will just use the modified object in the next step of the logic. That's what makes it appropriate to write code this way: we're still presenting an interface, so the caller doesn't have to worry about the details.
class Example:
def __init__(self):
self._words = ['hello']
def add_word(self):
self._words.append('world')
def display(self):
print(*self.words)
x = Example()
x.add_word()
x.display()
In the example, calling add_word gave information back to the top-level code - but instead of looking for it, we just go ahead and call display.3
See also: Passing variables between methods in Python?
Enclosing scope
This is a rare special case when using nested functions. There isn't a lot to say here - it works the same way as with the global scope, just using the nonlocal keyword rather than global.4
Global scope: Modifying a global
Generally speaking, it is a bad idea to change anything in the global scope after setting it up in the first place. It makes code harder to reason about, because anything that uses that global (aside from whatever was responsible for the change) now has a "hidden" source of input.
If you still want to do it, the syntax is straightforward:
words = ['hello']
def add_global_word():
words.append('world')
add_global_word() # `words` is changed
Global scope: Assigning to a new or existing global
This is actually a special case of modifying a global. I don't mean that assignment is a kind of modification (it isn't). I mean that when you assign a global name, Python automatically updates a dict that represents the global namespace. You can get that dict with globals(), and you can modify that dict and it will actually impact what global variables exist. (I.e., the return from globals() is the dictionary itself, not a copy.)5
But please don't. That's even worse of an idea than the previous one. If you really need to get the result from your function by assigning to a global variable, use the global keyword to tell Python that the name should be looked up in the global scope:
words = ['hello']
def replace_global_words():
global words
words = ['hello', 'world']
replace_global_words() # `words` is a new list with both words
Global scope: Assigning to or modifying an attribute of the function itself
This is a rare special case, but now that you've seen the other examples, the theory should be clear. In Python, functions are mutable (i.e. you can set attributes on them); and if we define a function at top level, it's in the global namespace. So this is really just modifying a global:
def set_own_words():
set_own_words.words = ['hello', 'world']
set_own_words()
print(*set_own_words.words)
We shouldn't really use this to send information to the caller. It has all the usual problems with globals, and it's even harder to understand. But it can be useful to set a function's attributes from within the function, in order for the function to remember something in between calls. (It's similar to how methods remember things in between calls by modifying self.) The functools standard library does this, for example in the cache implementation.
Builtin scope
This doesn't work. The builtin namespace doesn't contain any mutable objects, and you can't assign new builtin names (they'll go into the global namespace instead).
Some approaches that don't work in Python
Just calculating something before the function ends
In some other programming languages, there is some kind of hidden variable that automatically picks up the result of the last calculation, every time something is calculated; and if you reach the end of a function without returning anything, it gets returned. That doesn't work in Python. If you reach the end without returning anything, your function returns None.
Assigning to the function's name
In some other programming languages, you are allowed (or expected) to assign to a variable with the same name as the function; and at the end of the function, that value is returned. That still doesn't work in Python. If you reach the end without returning anything, your function still returns None.
def broken():
broken = 1
broken()
print(broken + 1) # causes a `TypeError`
It might seem like you can at least use the value that way, if you use the global keyword:
def subtly_broken():
global subtly_broken
subtly_broken = 1
subtly_broken()
print(subtly_broken + 1) # 2
But this, of course, is just a special case of assigning to a global. And there's a big problem with it - the same name can't refer to two things at once. By doing this, the function replaced its own name. So it will fail next time:
def subtly_broken():
global subtly_broken
subtly_broken = 1
subtly_broken()
subtly_broken() # causes a `TypeError`
Assigning to a parameter
Sometimes people expect to be able to assign to one of the function's parameters, and have it affect a variable that was used for the corresponding argument. However, this does not work:
def broken(words):
words = ['hello', 'world']
data = ['hello']
broken(data) # `data` does not change
Just like how Python returns values, not variables, it also passes values, not variables. words is a local name; by definition the calling code doesn't know anything about that namespace.
One of the working methods that we saw is to modify the passed-in list. That works because if the list itself changes, then it changes - it doesn't matter what name is used for it, or what part of the code uses that name. However, assigning a new list to words does not cause the existing list to change. It just makes words start being a name for a different list.
For more information, see How do I pass a variable by reference?.
1 At least, not for getting the value back. If you want to use keyword arguments, you need to know what the keyword names are. But generally, the point of functions is that they're an abstraction; you only need to know about their interface, and you don't need to think about what they're doing internally.
2 In 2.x, print is a statement rather than a function, so this doesn't make an example of calling another function directly. However, print foo() still works with 2.x's print statement, and so does print(foo()) (in this case, the extra parentheses are just ordinary grouping parentheses). Aside from that, 2.7 (the last 2.x version) has been unsupported since the beginning of 2020 - which was nearly a 5 year extension of the normal schedule. But then, this question was originally asked in 2010.
3Again: if the purpose of a method is to update the object, don't also return a value. Some people like to return self so that you can "chain" method calls; but in Python this is considered poor style. If you want that kind of "fluent" interface, then instead of writing methods that update self, write methods that create a new, modified instance of the class.
4 Except, of course, that if we're modifying a value rather than assigning, we don't need either keyword.
5 There's also a locals() that gives you a dict of local variables. However, this cannot be used to make new local variables - the behaviour is undefined in 2.x, and in 3.x the dict is created on the fly and assigning to it has no effect. Some of Python's optimizations depend on the local variables for a function being known ahead of time.
>>> def foo():
return 'hello world'
>>> x = foo()
>>> x
'hello world'
You can use global statement and then achieve what you want without returning value from
the function. For example you can do something like below:
def foo():
global x
x = "hello world"
foo()
print x
The above code will print "hello world".
But please be warned that usage of "global" is not a good idea at all and it is better to avoid usage that is shown in my example.
Also check this related discussion on about usage of global statement in Python.
First post on Overstack.
I'm new to Python and coding in general. I'm now learning about functions and presently looking at some examples that describe global variables on Python-Course.eu:
def f():
global s
print(s)
s = "Only in spring, but London is great as well!"
print(s)
s = "I am looking for a course in Paris!"
f()
print(s)
Which yields the output:
I am looking for a course in Paris!
Only in spring, but London is great as well!
Only in spring, but London is great as well!
So my understanding is that the global keyword changes the value of the global s variable to "Only in the spring, but London is great as well!"? Is that accurate?
If that is the case, if I assign global to a variable inside a function and don't assign a value to s outside the function, I get an error? Does this mean that the global keyword can only modify an already-assigned global variable?
For example, why can't I do this:
def f():
global s
s = "Only in spring, but London is great as well!"
print(s)
print(s)
f()
I've noticed that if I run the function first, it works. So does that mean that I have to run the function in order to assign the global variable?
This question isn't so much about trying to get a piece of code to work, but more of trying to understand how Python works and why. Thanks and I apologize if this question has been answered already. I just have not been able to find an answer. Thank you so very much in advance.
It's reasonably straightforward, as per the Python FAQ (with my emphasis):
In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.
So, if you assign to a variable anywhere within a function, that variables is considered local everywhere in the function, leading to issues such as:
def fn():
print(s) # Use before assignment, despite one s being
global s # set before calling.
s = 42
s = 7
fn()
That means, if you want to change a global, you need to mark it as global within the function.
If the only thing you're doing is using the global, that's not needed, but I tend to mark it anyway, just in case I later want to change it, and for clarity of intent (I use globals infrequently enough that this almost never comes up).
The reason your code sample doesn't work:
def f():
global s
s = "Only in spring, but London is great as well!"
print(s)
print(s)
f()
is because s does not exists at the point where you first print it (before the function call). Even if global did create the variable, that wouldn't have happened until the function was called, after that first print.
In any case, global does not create the variable, it merely indicates to the function to use the global when assigning. That's evident from the following code:
def f():
global s
print(s)
f()
which also complains about the non-existence of s.
In terms of creating globals, they come into existence when you assign to them. You can assign to them outside the function:
def fn():
print(s) # global because function doesn't assign to it.
# Unassigned here
s = 42
# Assigned here
fn()
or you can assign to them inside a function if they're marked global.
def fn():
global s
# Unassigned here
s = 42
# Assigned here
print(s) # global because you said so.
# Unassigned here
fn()
# Assigned here
In the absence of those cases, there is only a local:
def fn():
# Global unassigned here ...
s = 42
# ... and here
print(s) # local.
# Global unassigned here
fn()
# ... and here.
The first example you need to the look at the sequence of your code. Essentially the code will be in this order based on your sequence.
s = "I am looking for a course in Paris!"
print(s)
s = "Only in spring, but London is great as well!"
print(s)
print(s)
The second example why it failed as you haven't declared a variable s. global is just declaring scope of the variable s but you have to define it.
Regards
Paul
What you're missing is the order of statement execution here. As you noted, if you run the function first, it works:
# 1 definition of function f - don't parse or run the code inside here, just know it exists.
def f():
# 4 ... global statement
global s
# 5 assign a value to `s` (the global)
s = "Only in spring, but London is great as well!"
# 6 run print function with parameter `s`
print(s)
# 2 run print function with variable `s` as input. `s` doesn't exist. error.
print(s)
# if we pretend we skip the print
# 3 run the function `f`
f()
# After running `f()` `s` is now defined in the global scope
What you might find useful to examine this is checking out the contents of locals() and globals() those are builtin functions that return a dictionary of everything in the scope, so you can see how it changes based on assignments and function definitions.
I couldn't understand global and local variables in python, especially functions. I look lots of examples, but all of them don't explain in function calls clearly. My example code is below:
def called(d):
del d[0]
b=[1]
return b
def main():
a=[0,1,2,3]
print("first ", a)
c=called(a)
print("second ", a)
main()
Output:
first [0, 1, 2, 3]
second [1, 2, 3]
I expect that "a" is local variable at main. When I call the "called" function a is copyed to d. And d is local at "called". But reality is different. I solve the problem with a "called(a.copy)". But I want to understand the logic.
Python never implicitely copies anything, when you pass an object (and everything in Python is an object) to a function what you have in the function IS the object you passed in, so if you mutate it the change will be seen outside the function.
OTHO, parameters names are local to the function so rebinding a parameter within a function only change what object this name points to within the function, it has no impact on the object passed.
For more in-depth explanation the best reference is Ned Batchelder's article here.
Basically, a global variable is one that can be accessed anywhere, regardless of whether or not it is in a function (or anything else).
A local variable is one that solely exists within the function in question. You cannot declare or access it anywhere else. You could, however, make it global by explicitly including that in the function.
As far as I can see,'d' has not really been defined as a variable, but as a function parameter, as seen in called(d). This could've been changed to called(blabla) and your function would behave the same exact way if you, within the function, also changed d[0] to blabla[0].
What this means is that when you call that function, anything that has 'd' in it would be replaced by what you're calling it with. In this case, the parameter has been changed to the variable a, and the functions are then executed as you've stated.
In order to define a variable, you have to use '='.
Suppose I have a function like:
def foo():
x = 'hello world'
How do I get the function to return x, in such a way that I can use it as the input for another function or use the variable within the body of a program? I tried using return and then using the x variable in another function, but I get a NameError that way.
For the specific case of communicating information between methods in the same class, it is often best to store the information in self. See Passing variables between methods in Python? for details.
def foo():
x = 'hello world'
return x # return 'hello world' would do, too
foo()
print(x) # NameError - x is not defined outside the function
y = foo()
print(y) # this works
x = foo()
print(x) # this also works, and it's a completely different x than that inside
# foo()
z = bar(x) # of course, now you can use x as you want
z = bar(foo()) # but you don't have to
Effectively, there are two ways: directly and indirectly.
The direct way is to return a value from the function, as you tried, and let the calling code use that value. This is normally what you want. The natural, simple, direct, explicit way to get information back from a function is to return it. Broadly speaking, the purpose of a function is to compute a value, and return signifies "this is the value we computed; we are done here".
Directly using return
The main trick here is that return returns a value, not a variable. So return x does not enable the calling code to use x after calling the function, and does not modify any existing value that x had in the context of the call. (That's presumably why you got a NameError.)
After we use return in the function:
def example():
x = 'hello world'
return x
we need to write the calling code to use the return value:
result = example()
print(result)
The other key point here is that a call to a function is an expression, so we can use it the same way that we use, say, the result of an addition. Just as we may say result = 'hello ' + 'world', we may say result = foo(). After that, result is our own, local name for that string, and we can do whatever we want with it.
We can use the same name, x, if we want. Or we can use a different name. The calling code doesn't have to know anything about how the function is written, or what names it uses for things.1
We can use the value directly to call another function: for example, print(foo()).2 We can return the value directly: simply return 'hello world', without assigning to x. (Again: we are returning a value, not a variable.)
The function can only return once each time it is called. return terminates the function - again, we just determined the result of the calculation, so there is no reason to calculate any further. If we want to return multiple pieces of information, therefore, we will need to come up with a single object (in Python, "value" and "object" are effectively synonyms; this doesn't work out so well for some other languages.)
We can make a tuple right on the return line; or we can use a dictionary, a namedtuple (Python 2.6+), a types.simpleNamespace (Python 3.3+), a dataclass (Python 3.7+), or some other class (perhaps even one we write ourselves) to associate names with the values that are being returned; or we can accumulate values from a loop in a list; etc. etc. The possibilities are endless..
On the other hand, the function returns whether you like it or not (unless an exception is raised). If it reaches the end, it will implicitly return the special value None. You may or may not want to do it explicitly instead.
Indirect methods
Other than returning the result back to the caller directly, we can communicate it by modifying some existing object that the caller knows about. There are many ways to do that, but they're all variations on that same theme.
If you want the code to communicate information back this way, please just let it return None - don't also use the return value for something meaningful. That's how the built-in functionality works.
In order to modify that object, the called function also has to know about it, of course. That means, having a name for the object that can be looked up in a current scope. So, let's go through those in order:
Local scope: Modifying a passed-in argument
If one of our parameters is mutable, we can just mutate it, and rely on the caller to examine the change. This is usually not a great idea, because it can be hard to reason about the code. It looks like:
def called(mutable):
mutable.append('world')
def caller():
my_value = ['hello'] # a list with just that string
called(my_value)
# now it contains both strings
If the value is an instance of our own class, we could also assign to an attribute:
class Test:
def __init__(self, value):
self.value = value
def called(mutable):
mutable.value = 'world'
def caller():
test = Test('hello')
called(test)
# now test.value has changed
Assigning to an attribute does not work for built-in types, including object; and it might not work for some classes that explicitly prevent you from doing it.
Local scope: Modifying self, in a method
We already have an example of this above: setting self.value in the Test.__init__ code. This is a special case of modifying a passed-in argument; but it's part of how classes work in Python, and something we're expected to do. Normally, when we do this, the calling won't actually check for changes to self - it will just use the modified object in the next step of the logic. That's what makes it appropriate to write code this way: we're still presenting an interface, so the caller doesn't have to worry about the details.
class Example:
def __init__(self):
self._words = ['hello']
def add_word(self):
self._words.append('world')
def display(self):
print(*self.words)
x = Example()
x.add_word()
x.display()
In the example, calling add_word gave information back to the top-level code - but instead of looking for it, we just go ahead and call display.3
See also: Passing variables between methods in Python?
Enclosing scope
This is a rare special case when using nested functions. There isn't a lot to say here - it works the same way as with the global scope, just using the nonlocal keyword rather than global.4
Global scope: Modifying a global
Generally speaking, it is a bad idea to change anything in the global scope after setting it up in the first place. It makes code harder to reason about, because anything that uses that global (aside from whatever was responsible for the change) now has a "hidden" source of input.
If you still want to do it, the syntax is straightforward:
words = ['hello']
def add_global_word():
words.append('world')
add_global_word() # `words` is changed
Global scope: Assigning to a new or existing global
This is actually a special case of modifying a global. I don't mean that assignment is a kind of modification (it isn't). I mean that when you assign a global name, Python automatically updates a dict that represents the global namespace. You can get that dict with globals(), and you can modify that dict and it will actually impact what global variables exist. (I.e., the return from globals() is the dictionary itself, not a copy.)5
But please don't. That's even worse of an idea than the previous one. If you really need to get the result from your function by assigning to a global variable, use the global keyword to tell Python that the name should be looked up in the global scope:
words = ['hello']
def replace_global_words():
global words
words = ['hello', 'world']
replace_global_words() # `words` is a new list with both words
Global scope: Assigning to or modifying an attribute of the function itself
This is a rare special case, but now that you've seen the other examples, the theory should be clear. In Python, functions are mutable (i.e. you can set attributes on them); and if we define a function at top level, it's in the global namespace. So this is really just modifying a global:
def set_own_words():
set_own_words.words = ['hello', 'world']
set_own_words()
print(*set_own_words.words)
We shouldn't really use this to send information to the caller. It has all the usual problems with globals, and it's even harder to understand. But it can be useful to set a function's attributes from within the function, in order for the function to remember something in between calls. (It's similar to how methods remember things in between calls by modifying self.) The functools standard library does this, for example in the cache implementation.
Builtin scope
This doesn't work. The builtin namespace doesn't contain any mutable objects, and you can't assign new builtin names (they'll go into the global namespace instead).
Some approaches that don't work in Python
Just calculating something before the function ends
In some other programming languages, there is some kind of hidden variable that automatically picks up the result of the last calculation, every time something is calculated; and if you reach the end of a function without returning anything, it gets returned. That doesn't work in Python. If you reach the end without returning anything, your function returns None.
Assigning to the function's name
In some other programming languages, you are allowed (or expected) to assign to a variable with the same name as the function; and at the end of the function, that value is returned. That still doesn't work in Python. If you reach the end without returning anything, your function still returns None.
def broken():
broken = 1
broken()
print(broken + 1) # causes a `TypeError`
It might seem like you can at least use the value that way, if you use the global keyword:
def subtly_broken():
global subtly_broken
subtly_broken = 1
subtly_broken()
print(subtly_broken + 1) # 2
But this, of course, is just a special case of assigning to a global. And there's a big problem with it - the same name can't refer to two things at once. By doing this, the function replaced its own name. So it will fail next time:
def subtly_broken():
global subtly_broken
subtly_broken = 1
subtly_broken()
subtly_broken() # causes a `TypeError`
Assigning to a parameter
Sometimes people expect to be able to assign to one of the function's parameters, and have it affect a variable that was used for the corresponding argument. However, this does not work:
def broken(words):
words = ['hello', 'world']
data = ['hello']
broken(data) # `data` does not change
Just like how Python returns values, not variables, it also passes values, not variables. words is a local name; by definition the calling code doesn't know anything about that namespace.
One of the working methods that we saw is to modify the passed-in list. That works because if the list itself changes, then it changes - it doesn't matter what name is used for it, or what part of the code uses that name. However, assigning a new list to words does not cause the existing list to change. It just makes words start being a name for a different list.
For more information, see How do I pass a variable by reference?.
1 At least, not for getting the value back. If you want to use keyword arguments, you need to know what the keyword names are. But generally, the point of functions is that they're an abstraction; you only need to know about their interface, and you don't need to think about what they're doing internally.
2 In 2.x, print is a statement rather than a function, so this doesn't make an example of calling another function directly. However, print foo() still works with 2.x's print statement, and so does print(foo()) (in this case, the extra parentheses are just ordinary grouping parentheses). Aside from that, 2.7 (the last 2.x version) has been unsupported since the beginning of 2020 - which was nearly a 5 year extension of the normal schedule. But then, this question was originally asked in 2010.
3Again: if the purpose of a method is to update the object, don't also return a value. Some people like to return self so that you can "chain" method calls; but in Python this is considered poor style. If you want that kind of "fluent" interface, then instead of writing methods that update self, write methods that create a new, modified instance of the class.
4 Except, of course, that if we're modifying a value rather than assigning, we don't need either keyword.
5 There's also a locals() that gives you a dict of local variables. However, this cannot be used to make new local variables - the behaviour is undefined in 2.x, and in 3.x the dict is created on the fly and assigning to it has no effect. Some of Python's optimizations depend on the local variables for a function being known ahead of time.
>>> def foo():
return 'hello world'
>>> x = foo()
>>> x
'hello world'
You can use global statement and then achieve what you want without returning value from
the function. For example you can do something like below:
def foo():
global x
x = "hello world"
foo()
print x
The above code will print "hello world".
But please be warned that usage of "global" is not a good idea at all and it is better to avoid usage that is shown in my example.
Also check this related discussion on about usage of global statement in Python.
Suppose I have a function like:
def foo():
x = 'hello world'
How do I get the function to return x, in such a way that I can use it as the input for another function or use the variable within the body of a program? I tried using return and then using the x variable in another function, but I get a NameError that way.
For the specific case of communicating information between methods in the same class, it is often best to store the information in self. See Passing variables between methods in Python? for details.
def foo():
x = 'hello world'
return x # return 'hello world' would do, too
foo()
print(x) # NameError - x is not defined outside the function
y = foo()
print(y) # this works
x = foo()
print(x) # this also works, and it's a completely different x than that inside
# foo()
z = bar(x) # of course, now you can use x as you want
z = bar(foo()) # but you don't have to
Effectively, there are two ways: directly and indirectly.
The direct way is to return a value from the function, as you tried, and let the calling code use that value. This is normally what you want. The natural, simple, direct, explicit way to get information back from a function is to return it. Broadly speaking, the purpose of a function is to compute a value, and return signifies "this is the value we computed; we are done here".
Directly using return
The main trick here is that return returns a value, not a variable. So return x does not enable the calling code to use x after calling the function, and does not modify any existing value that x had in the context of the call. (That's presumably why you got a NameError.)
After we use return in the function:
def example():
x = 'hello world'
return x
we need to write the calling code to use the return value:
result = example()
print(result)
The other key point here is that a call to a function is an expression, so we can use it the same way that we use, say, the result of an addition. Just as we may say result = 'hello ' + 'world', we may say result = foo(). After that, result is our own, local name for that string, and we can do whatever we want with it.
We can use the same name, x, if we want. Or we can use a different name. The calling code doesn't have to know anything about how the function is written, or what names it uses for things.1
We can use the value directly to call another function: for example, print(foo()).2 We can return the value directly: simply return 'hello world', without assigning to x. (Again: we are returning a value, not a variable.)
The function can only return once each time it is called. return terminates the function - again, we just determined the result of the calculation, so there is no reason to calculate any further. If we want to return multiple pieces of information, therefore, we will need to come up with a single object (in Python, "value" and "object" are effectively synonyms; this doesn't work out so well for some other languages.)
We can make a tuple right on the return line; or we can use a dictionary, a namedtuple (Python 2.6+), a types.simpleNamespace (Python 3.3+), a dataclass (Python 3.7+), or some other class (perhaps even one we write ourselves) to associate names with the values that are being returned; or we can accumulate values from a loop in a list; etc. etc. The possibilities are endless..
On the other hand, the function returns whether you like it or not (unless an exception is raised). If it reaches the end, it will implicitly return the special value None. You may or may not want to do it explicitly instead.
Indirect methods
Other than returning the result back to the caller directly, we can communicate it by modifying some existing object that the caller knows about. There are many ways to do that, but they're all variations on that same theme.
If you want the code to communicate information back this way, please just let it return None - don't also use the return value for something meaningful. That's how the built-in functionality works.
In order to modify that object, the called function also has to know about it, of course. That means, having a name for the object that can be looked up in a current scope. So, let's go through those in order:
Local scope: Modifying a passed-in argument
If one of our parameters is mutable, we can just mutate it, and rely on the caller to examine the change. This is usually not a great idea, because it can be hard to reason about the code. It looks like:
def called(mutable):
mutable.append('world')
def caller():
my_value = ['hello'] # a list with just that string
called(my_value)
# now it contains both strings
If the value is an instance of our own class, we could also assign to an attribute:
class Test:
def __init__(self, value):
self.value = value
def called(mutable):
mutable.value = 'world'
def caller():
test = Test('hello')
called(test)
# now test.value has changed
Assigning to an attribute does not work for built-in types, including object; and it might not work for some classes that explicitly prevent you from doing it.
Local scope: Modifying self, in a method
We already have an example of this above: setting self.value in the Test.__init__ code. This is a special case of modifying a passed-in argument; but it's part of how classes work in Python, and something we're expected to do. Normally, when we do this, the calling won't actually check for changes to self - it will just use the modified object in the next step of the logic. That's what makes it appropriate to write code this way: we're still presenting an interface, so the caller doesn't have to worry about the details.
class Example:
def __init__(self):
self._words = ['hello']
def add_word(self):
self._words.append('world')
def display(self):
print(*self.words)
x = Example()
x.add_word()
x.display()
In the example, calling add_word gave information back to the top-level code - but instead of looking for it, we just go ahead and call display.3
See also: Passing variables between methods in Python?
Enclosing scope
This is a rare special case when using nested functions. There isn't a lot to say here - it works the same way as with the global scope, just using the nonlocal keyword rather than global.4
Global scope: Modifying a global
Generally speaking, it is a bad idea to change anything in the global scope after setting it up in the first place. It makes code harder to reason about, because anything that uses that global (aside from whatever was responsible for the change) now has a "hidden" source of input.
If you still want to do it, the syntax is straightforward:
words = ['hello']
def add_global_word():
words.append('world')
add_global_word() # `words` is changed
Global scope: Assigning to a new or existing global
This is actually a special case of modifying a global. I don't mean that assignment is a kind of modification (it isn't). I mean that when you assign a global name, Python automatically updates a dict that represents the global namespace. You can get that dict with globals(), and you can modify that dict and it will actually impact what global variables exist. (I.e., the return from globals() is the dictionary itself, not a copy.)5
But please don't. That's even worse of an idea than the previous one. If you really need to get the result from your function by assigning to a global variable, use the global keyword to tell Python that the name should be looked up in the global scope:
words = ['hello']
def replace_global_words():
global words
words = ['hello', 'world']
replace_global_words() # `words` is a new list with both words
Global scope: Assigning to or modifying an attribute of the function itself
This is a rare special case, but now that you've seen the other examples, the theory should be clear. In Python, functions are mutable (i.e. you can set attributes on them); and if we define a function at top level, it's in the global namespace. So this is really just modifying a global:
def set_own_words():
set_own_words.words = ['hello', 'world']
set_own_words()
print(*set_own_words.words)
We shouldn't really use this to send information to the caller. It has all the usual problems with globals, and it's even harder to understand. But it can be useful to set a function's attributes from within the function, in order for the function to remember something in between calls. (It's similar to how methods remember things in between calls by modifying self.) The functools standard library does this, for example in the cache implementation.
Builtin scope
This doesn't work. The builtin namespace doesn't contain any mutable objects, and you can't assign new builtin names (they'll go into the global namespace instead).
Some approaches that don't work in Python
Just calculating something before the function ends
In some other programming languages, there is some kind of hidden variable that automatically picks up the result of the last calculation, every time something is calculated; and if you reach the end of a function without returning anything, it gets returned. That doesn't work in Python. If you reach the end without returning anything, your function returns None.
Assigning to the function's name
In some other programming languages, you are allowed (or expected) to assign to a variable with the same name as the function; and at the end of the function, that value is returned. That still doesn't work in Python. If you reach the end without returning anything, your function still returns None.
def broken():
broken = 1
broken()
print(broken + 1) # causes a `TypeError`
It might seem like you can at least use the value that way, if you use the global keyword:
def subtly_broken():
global subtly_broken
subtly_broken = 1
subtly_broken()
print(subtly_broken + 1) # 2
But this, of course, is just a special case of assigning to a global. And there's a big problem with it - the same name can't refer to two things at once. By doing this, the function replaced its own name. So it will fail next time:
def subtly_broken():
global subtly_broken
subtly_broken = 1
subtly_broken()
subtly_broken() # causes a `TypeError`
Assigning to a parameter
Sometimes people expect to be able to assign to one of the function's parameters, and have it affect a variable that was used for the corresponding argument. However, this does not work:
def broken(words):
words = ['hello', 'world']
data = ['hello']
broken(data) # `data` does not change
Just like how Python returns values, not variables, it also passes values, not variables. words is a local name; by definition the calling code doesn't know anything about that namespace.
One of the working methods that we saw is to modify the passed-in list. That works because if the list itself changes, then it changes - it doesn't matter what name is used for it, or what part of the code uses that name. However, assigning a new list to words does not cause the existing list to change. It just makes words start being a name for a different list.
For more information, see How do I pass a variable by reference?.
1 At least, not for getting the value back. If you want to use keyword arguments, you need to know what the keyword names are. But generally, the point of functions is that they're an abstraction; you only need to know about their interface, and you don't need to think about what they're doing internally.
2 In 2.x, print is a statement rather than a function, so this doesn't make an example of calling another function directly. However, print foo() still works with 2.x's print statement, and so does print(foo()) (in this case, the extra parentheses are just ordinary grouping parentheses). Aside from that, 2.7 (the last 2.x version) has been unsupported since the beginning of 2020 - which was nearly a 5 year extension of the normal schedule. But then, this question was originally asked in 2010.
3Again: if the purpose of a method is to update the object, don't also return a value. Some people like to return self so that you can "chain" method calls; but in Python this is considered poor style. If you want that kind of "fluent" interface, then instead of writing methods that update self, write methods that create a new, modified instance of the class.
4 Except, of course, that if we're modifying a value rather than assigning, we don't need either keyword.
5 There's also a locals() that gives you a dict of local variables. However, this cannot be used to make new local variables - the behaviour is undefined in 2.x, and in 3.x the dict is created on the fly and assigning to it has no effect. Some of Python's optimizations depend on the local variables for a function being known ahead of time.
>>> def foo():
return 'hello world'
>>> x = foo()
>>> x
'hello world'
You can use global statement and then achieve what you want without returning value from
the function. For example you can do something like below:
def foo():
global x
x = "hello world"
foo()
print x
The above code will print "hello world".
But please be warned that usage of "global" is not a good idea at all and it is better to avoid usage that is shown in my example.
Also check this related discussion on about usage of global statement in Python.