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Is it possible to forward-declare a function in Python? I want to sort a list using my own cmp function before it is declared.
print "\n".join([str(bla) for bla in sorted(mylist, cmp = cmp_configs)])
I've put the definition of cmp_configs method after the invocation. It fails with this error:
NameError: name 'cmp_configs' is not defined
Is there any way to "declare" cmp_configs method before it's used?
Sometimes, it is difficult to reorganize code to avoid this problem. For instance, when implementing some forms of recursion:
def spam():
if end_condition():
return end_result()
else:
return eggs()
def eggs():
if end_condition():
return end_result()
else:
return spam()
Where end_condition and end_result have been previously defined.
Is the only solution to reorganize the code and always put definitions before invocations?
Wrap the invocation into a function of its own so that
foo()
def foo():
print "Hi!"
will break, but
def bar():
foo()
def foo():
print "Hi!"
bar()
will work properly.
The general rule in Python is that a function should be defined before its usage, which does not necessarily mean it needs to be higher in the code.
If you kick-start your script through the following:
if __name__=="__main__":
main()
then you probably do not have to worry about things like "forward declaration". You see, the interpreter would go loading up all your functions and then start your main() function. Of course, make sure you have all the imports correct too ;-)
Come to think of it, I've never heard such a thing as "forward declaration" in python... but then again, I might be wrong ;-)
If you don't want to define a function before it's used, and defining it afterwards is impossible, what about defining it in some other module?
Technically you still define it first, but it's clean.
You could create a recursion like the following:
def foo():
bar()
def bar():
foo()
Python's functions are anonymous just like values are anonymous, yet they can be bound to a name.
In the above code, foo() does not call a function with the name foo, it calls a function that happens to be bound to the name foo at the point the call is made. It is possible to redefine foo somewhere else, and bar would then call the new function.
Your problem cannot be solved because it's like asking to get a variable which has not been declared.
I apologize for reviving this thread, but there was a strategy not discussed here which may be applicable.
Using reflection it is possible to do something akin to forward declaration. For instance lets say you have a section of code that looks like this:
# We want to call a function called 'foo', but it hasn't been defined yet.
function_name = 'foo'
# Calling at this point would produce an error
# Here is the definition
def foo():
bar()
# Note that at this point the function is defined
# Time for some reflection...
globals()[function_name]()
So in this way we have determined what function we want to call before it is actually defined, effectively a forward declaration. In python the statement globals()[function_name]() is the same as foo() if function_name = 'foo' for the reasons discussed above, since python must lookup each function before calling it. If one were to use the timeit module to see how these two statements compare, they have the exact same computational cost.
Of course the example here is very useless, but if one were to have a complex structure which needed to execute a function, but must be declared before (or structurally it makes little sense to have it afterwards), one can just store a string and try to call the function later.
If the call to cmp_configs is inside its own function definition, you should be fine. I'll give an example.
def a():
b() # b() hasn't been defined yet, but that's fine because at this point, we're not
# actually calling it. We're just defining what should happen when a() is called.
a() # This call fails, because b() hasn't been defined yet,
# and thus trying to run a() fails.
def b():
print "hi"
a() # This call succeeds because everything has been defined.
In general, putting your code inside functions (such as main()) will resolve your problem; just call main() at the end of the file.
There is no such thing in python like forward declaration. You just have to make sure that your function is declared before it is needed.
Note that the body of a function isn't interpreted until the function is executed.
Consider the following example:
def a():
b() # won't be resolved until a is invoked.
def b():
print "hello"
a() # here b is already defined so this line won't fail.
You can think that a body of a function is just another script that will be interpreted once you call the function.
Sometimes an algorithm is easiest to understand top-down, starting with the overall structure and drilling down into the details.
You can do so without forward declarations:
def main():
make_omelet()
eat()
def make_omelet():
break_eggs()
whisk()
fry()
def break_eggs():
for egg in carton:
break(egg)
# ...
main()
# declare a fake function (prototype) with no body
def foo(): pass
def bar():
# use the prototype however you see fit
print(foo(), "world!")
# define the actual function (overwriting the prototype)
def foo():
return "Hello,"
bar()
Output:
Hello, world!
No, I don't believe there is any way to forward-declare a function in Python.
Imagine you are the Python interpreter. When you get to the line
print "\n".join([str(bla) for bla in sorted(mylist, cmp = cmp_configs)])
either you know what cmp_configs is or you don't. In order to proceed, you have to
know cmp_configs. It doesn't matter if there is recursion.
You can't forward-declare a function in Python. If you have logic executing before you've defined functions, you've probably got a problem anyways. Put your action in an if __name__ == '__main__' at the end of your script (by executing a function you name "main" if it's non-trivial) and your code will be more modular and you'll be able to use it as a module if you ever need to.
Also, replace that list comprehension with a generator express (i.e., print "\n".join(str(bla) for bla in sorted(mylist, cmp=cmp_configs)))
Also, don't use cmp, which is deprecated. Use key and provide a less-than function.
Import the file itself. Assuming the file is called test.py:
import test
if __name__=='__main__':
test.func()
else:
def func():
print('Func worked')
TL;DR: Python does not need forward declarations. Simply put your function calls inside function def definitions, and you'll be fine.
def foo(count):
print("foo "+str(count))
if(count>0):
bar(count-1)
def bar(count):
print("bar "+str(count))
if(count>0):
foo(count-1)
foo(3)
print("Finished.")
recursive function definitions, perfectly successfully gives:
foo 3
bar 2
foo 1
bar 0
Finished.
However,
bug(13)
def bug(count):
print("bug never runs "+str(count))
print("Does not print this.")
breaks at the top-level invocation of a function that hasn't been defined yet, and gives:
Traceback (most recent call last):
File "./test1.py", line 1, in <module>
bug(13)
NameError: name 'bug' is not defined
Python is an interpreted language, like Lisp. It has no type checking, only run-time function invocations, which succeed if the function name has been bound and fail if it's unbound.
Critically, a function def definition does not execute any of the funcalls inside its lines, it simply declares what the function body is going to consist of. Again, it doesn't even do type checking. So we can do this:
def uncalled():
wild_eyed_undefined_function()
print("I'm not invoked!")
print("Only run this one line.")
and it runs perfectly fine (!), with output
Only run this one line.
The key is the difference between definitions and invocations.
The interpreter executes everything that comes in at the top level, which means it tries to invoke it. If it's not inside a definition.
Your code is running into trouble because you attempted to invoke a function, at the top level in this case, before it was bound.
The solution is to put your non-top-level function invocations inside a function definition, then call that function sometime much later.
The business about "if __ main __" is an idiom based on this principle, but you have to understand why, instead of simply blindly following it.
There are certainly much more advanced topics concerning lambda functions and rebinding function names dynamically, but these are not what the OP was asking for. In addition, they can be solved using these same principles: (1) defs define a function, they do not invoke their lines; (2) you get in trouble when you invoke a function symbol that's unbound.
Python does not support forward declarations, but common workaround for this is use of the the following condition at the end of your script/code:
if __name__ == '__main__': main()
With this it will read entire file first and then evaluate condition and call main() function which will be able to call any forward declared function as it already read the entire file first. This condition leverages special variable __name__ which returns __main__ value whenever we run Python code from current file (when code was imported as a module, then __name__ returns module name).
"just reorganize my code so that I don't have this problem." Correct. Easy to do. Always works.
You can always provide the function prior to it's reference.
"However, there are cases when this is probably unavoidable, for instance when implementing some forms of recursion"
Can't see how that's even remotely possible. Please provide an example of a place where you cannot define the function prior to it's use.
Now wait a minute. When your module reaches the print statement in your example, before cmp_configs has been defined, what exactly is it that you expect it to do?
If your posting of a question using print is really trying to represent something like this:
fn = lambda mylist:"\n".join([str(bla)
for bla in sorted(mylist, cmp = cmp_configs)])
then there is no requirement to define cmp_configs before executing this statement, just define it later in the code and all will be well.
Now if you are trying to reference cmp_configs as a default value of an argument to the lambda, then this is a different story:
fn = lambda mylist,cmp_configs=cmp_configs : \
"\n".join([str(bla) for bla in sorted(mylist, cmp = cmp_configs)])
Now you need a cmp_configs variable defined before you reach this line.
[EDIT - this next part turns out not to be correct, since the default argument value will get assigned when the function is compiled, and that value will be used even if you change the value of cmp_configs later.]
Fortunately, Python being so type-accommodating as it is, does not care what you define as cmp_configs, so you could just preface with this statement:
cmp_configs = None
And the compiler will be happy. Just be sure to declare the real cmp_configs before you ever invoke fn.
Python technically has support for forward declaration.
if you define a function/class then set the body to pass, it will have an empty entry in the global table.
you can then "redefine" the function/class later on to implement the function/class.
unlike c/c++ forward declaration though, this does not work from outside the scope (i.e. another file) as they have their own "global" namespace
example:
def foo(): pass
foo()
def foo(): print("FOOOOO")
foo()
foo is declared both times
however the first time foo is called it does not do anything as the body is just pass
but the second time foo is called. it executes the new body of print("FOOOOO")
but again. note that this does not fix circular dependancies. this is because files have their own name and have their own definitions of functions
example 2:
class bar: pass
print(bar)
this prints <class '__main__.bar'> but if it was declared in another file it would be <class 'otherfile.foo'>
i know this post is old, but i though that this answer would be useful to anyone who keeps finding this post after the many years it has been posted for
One way is to create a handler function. Define the handler early on, and put the handler below all the methods you need to call.
Then when you invoke the handler method to call your functions, they will always be available.
The handler could take an argument nameOfMethodToCall. Then uses a bunch of if statements to call the right method.
This would solve your issue.
def foo():
print("foo")
#take input
nextAction=input('What would you like to do next?:')
return nextAction
def bar():
print("bar")
nextAction=input('What would you like to do next?:')
return nextAction
def handler(action):
if(action=="foo"):
nextAction = foo()
elif(action=="bar"):
nextAction = bar()
else:
print("You entered invalid input, defaulting to bar")
nextAction = "bar"
return nextAction
nextAction=input('What would you like to do next?:')
while 1:
nextAction = handler(nextAction)
Is it possible to forward-declare a function in Python? I want to sort a list using my own cmp function before it is declared.
print "\n".join([str(bla) for bla in sorted(mylist, cmp = cmp_configs)])
I've put the definition of cmp_configs method after the invocation. It fails with this error:
NameError: name 'cmp_configs' is not defined
Is there any way to "declare" cmp_configs method before it's used?
Sometimes, it is difficult to reorganize code to avoid this problem. For instance, when implementing some forms of recursion:
def spam():
if end_condition():
return end_result()
else:
return eggs()
def eggs():
if end_condition():
return end_result()
else:
return spam()
Where end_condition and end_result have been previously defined.
Is the only solution to reorganize the code and always put definitions before invocations?
Wrap the invocation into a function of its own so that
foo()
def foo():
print "Hi!"
will break, but
def bar():
foo()
def foo():
print "Hi!"
bar()
will work properly.
The general rule in Python is that a function should be defined before its usage, which does not necessarily mean it needs to be higher in the code.
If you kick-start your script through the following:
if __name__=="__main__":
main()
then you probably do not have to worry about things like "forward declaration". You see, the interpreter would go loading up all your functions and then start your main() function. Of course, make sure you have all the imports correct too ;-)
Come to think of it, I've never heard such a thing as "forward declaration" in python... but then again, I might be wrong ;-)
If you don't want to define a function before it's used, and defining it afterwards is impossible, what about defining it in some other module?
Technically you still define it first, but it's clean.
You could create a recursion like the following:
def foo():
bar()
def bar():
foo()
Python's functions are anonymous just like values are anonymous, yet they can be bound to a name.
In the above code, foo() does not call a function with the name foo, it calls a function that happens to be bound to the name foo at the point the call is made. It is possible to redefine foo somewhere else, and bar would then call the new function.
Your problem cannot be solved because it's like asking to get a variable which has not been declared.
I apologize for reviving this thread, but there was a strategy not discussed here which may be applicable.
Using reflection it is possible to do something akin to forward declaration. For instance lets say you have a section of code that looks like this:
# We want to call a function called 'foo', but it hasn't been defined yet.
function_name = 'foo'
# Calling at this point would produce an error
# Here is the definition
def foo():
bar()
# Note that at this point the function is defined
# Time for some reflection...
globals()[function_name]()
So in this way we have determined what function we want to call before it is actually defined, effectively a forward declaration. In python the statement globals()[function_name]() is the same as foo() if function_name = 'foo' for the reasons discussed above, since python must lookup each function before calling it. If one were to use the timeit module to see how these two statements compare, they have the exact same computational cost.
Of course the example here is very useless, but if one were to have a complex structure which needed to execute a function, but must be declared before (or structurally it makes little sense to have it afterwards), one can just store a string and try to call the function later.
If the call to cmp_configs is inside its own function definition, you should be fine. I'll give an example.
def a():
b() # b() hasn't been defined yet, but that's fine because at this point, we're not
# actually calling it. We're just defining what should happen when a() is called.
a() # This call fails, because b() hasn't been defined yet,
# and thus trying to run a() fails.
def b():
print "hi"
a() # This call succeeds because everything has been defined.
In general, putting your code inside functions (such as main()) will resolve your problem; just call main() at the end of the file.
There is no such thing in python like forward declaration. You just have to make sure that your function is declared before it is needed.
Note that the body of a function isn't interpreted until the function is executed.
Consider the following example:
def a():
b() # won't be resolved until a is invoked.
def b():
print "hello"
a() # here b is already defined so this line won't fail.
You can think that a body of a function is just another script that will be interpreted once you call the function.
Sometimes an algorithm is easiest to understand top-down, starting with the overall structure and drilling down into the details.
You can do so without forward declarations:
def main():
make_omelet()
eat()
def make_omelet():
break_eggs()
whisk()
fry()
def break_eggs():
for egg in carton:
break(egg)
# ...
main()
# declare a fake function (prototype) with no body
def foo(): pass
def bar():
# use the prototype however you see fit
print(foo(), "world!")
# define the actual function (overwriting the prototype)
def foo():
return "Hello,"
bar()
Output:
Hello, world!
No, I don't believe there is any way to forward-declare a function in Python.
Imagine you are the Python interpreter. When you get to the line
print "\n".join([str(bla) for bla in sorted(mylist, cmp = cmp_configs)])
either you know what cmp_configs is or you don't. In order to proceed, you have to
know cmp_configs. It doesn't matter if there is recursion.
You can't forward-declare a function in Python. If you have logic executing before you've defined functions, you've probably got a problem anyways. Put your action in an if __name__ == '__main__' at the end of your script (by executing a function you name "main" if it's non-trivial) and your code will be more modular and you'll be able to use it as a module if you ever need to.
Also, replace that list comprehension with a generator express (i.e., print "\n".join(str(bla) for bla in sorted(mylist, cmp=cmp_configs)))
Also, don't use cmp, which is deprecated. Use key and provide a less-than function.
Import the file itself. Assuming the file is called test.py:
import test
if __name__=='__main__':
test.func()
else:
def func():
print('Func worked')
TL;DR: Python does not need forward declarations. Simply put your function calls inside function def definitions, and you'll be fine.
def foo(count):
print("foo "+str(count))
if(count>0):
bar(count-1)
def bar(count):
print("bar "+str(count))
if(count>0):
foo(count-1)
foo(3)
print("Finished.")
recursive function definitions, perfectly successfully gives:
foo 3
bar 2
foo 1
bar 0
Finished.
However,
bug(13)
def bug(count):
print("bug never runs "+str(count))
print("Does not print this.")
breaks at the top-level invocation of a function that hasn't been defined yet, and gives:
Traceback (most recent call last):
File "./test1.py", line 1, in <module>
bug(13)
NameError: name 'bug' is not defined
Python is an interpreted language, like Lisp. It has no type checking, only run-time function invocations, which succeed if the function name has been bound and fail if it's unbound.
Critically, a function def definition does not execute any of the funcalls inside its lines, it simply declares what the function body is going to consist of. Again, it doesn't even do type checking. So we can do this:
def uncalled():
wild_eyed_undefined_function()
print("I'm not invoked!")
print("Only run this one line.")
and it runs perfectly fine (!), with output
Only run this one line.
The key is the difference between definitions and invocations.
The interpreter executes everything that comes in at the top level, which means it tries to invoke it. If it's not inside a definition.
Your code is running into trouble because you attempted to invoke a function, at the top level in this case, before it was bound.
The solution is to put your non-top-level function invocations inside a function definition, then call that function sometime much later.
The business about "if __ main __" is an idiom based on this principle, but you have to understand why, instead of simply blindly following it.
There are certainly much more advanced topics concerning lambda functions and rebinding function names dynamically, but these are not what the OP was asking for. In addition, they can be solved using these same principles: (1) defs define a function, they do not invoke their lines; (2) you get in trouble when you invoke a function symbol that's unbound.
Python does not support forward declarations, but common workaround for this is use of the the following condition at the end of your script/code:
if __name__ == '__main__': main()
With this it will read entire file first and then evaluate condition and call main() function which will be able to call any forward declared function as it already read the entire file first. This condition leverages special variable __name__ which returns __main__ value whenever we run Python code from current file (when code was imported as a module, then __name__ returns module name).
"just reorganize my code so that I don't have this problem." Correct. Easy to do. Always works.
You can always provide the function prior to it's reference.
"However, there are cases when this is probably unavoidable, for instance when implementing some forms of recursion"
Can't see how that's even remotely possible. Please provide an example of a place where you cannot define the function prior to it's use.
Now wait a minute. When your module reaches the print statement in your example, before cmp_configs has been defined, what exactly is it that you expect it to do?
If your posting of a question using print is really trying to represent something like this:
fn = lambda mylist:"\n".join([str(bla)
for bla in sorted(mylist, cmp = cmp_configs)])
then there is no requirement to define cmp_configs before executing this statement, just define it later in the code and all will be well.
Now if you are trying to reference cmp_configs as a default value of an argument to the lambda, then this is a different story:
fn = lambda mylist,cmp_configs=cmp_configs : \
"\n".join([str(bla) for bla in sorted(mylist, cmp = cmp_configs)])
Now you need a cmp_configs variable defined before you reach this line.
[EDIT - this next part turns out not to be correct, since the default argument value will get assigned when the function is compiled, and that value will be used even if you change the value of cmp_configs later.]
Fortunately, Python being so type-accommodating as it is, does not care what you define as cmp_configs, so you could just preface with this statement:
cmp_configs = None
And the compiler will be happy. Just be sure to declare the real cmp_configs before you ever invoke fn.
Python technically has support for forward declaration.
if you define a function/class then set the body to pass, it will have an empty entry in the global table.
you can then "redefine" the function/class later on to implement the function/class.
unlike c/c++ forward declaration though, this does not work from outside the scope (i.e. another file) as they have their own "global" namespace
example:
def foo(): pass
foo()
def foo(): print("FOOOOO")
foo()
foo is declared both times
however the first time foo is called it does not do anything as the body is just pass
but the second time foo is called. it executes the new body of print("FOOOOO")
but again. note that this does not fix circular dependancies. this is because files have their own name and have their own definitions of functions
example 2:
class bar: pass
print(bar)
this prints <class '__main__.bar'> but if it was declared in another file it would be <class 'otherfile.foo'>
i know this post is old, but i though that this answer would be useful to anyone who keeps finding this post after the many years it has been posted for
One way is to create a handler function. Define the handler early on, and put the handler below all the methods you need to call.
Then when you invoke the handler method to call your functions, they will always be available.
The handler could take an argument nameOfMethodToCall. Then uses a bunch of if statements to call the right method.
This would solve your issue.
def foo():
print("foo")
#take input
nextAction=input('What would you like to do next?:')
return nextAction
def bar():
print("bar")
nextAction=input('What would you like to do next?:')
return nextAction
def handler(action):
if(action=="foo"):
nextAction = foo()
elif(action=="bar"):
nextAction = bar()
else:
print("You entered invalid input, defaulting to bar")
nextAction = "bar"
return nextAction
nextAction=input('What would you like to do next?:')
while 1:
nextAction = handler(nextAction)
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Let's say I have the following script, test.py:
import my_library
bar = 12
def foo():
nested_bar = 21
my_library.do_things()
def nested_foo():
nested_bar += 11
not_a_variable += 1
{$ invalid_syntax
bar = 13
foo()
bar = 14
I'm curious as to what exactly happens when I run python test.py. Obviously Python doesn't just read programs line-by-line - otherwise it wouldn't catch syntax errors before actually executing the program. But this makes the workings of the interpreter seem somewhat nebulous. I was wondering if someone would help clear things up for me. In particular, I would like to know:
At what point does Python realize there is a syntax error on line 13?
At what point does Python read the nested functions and add them to the scope of foo?
Similarly, how does Python add the function foo to its namespace when it encounters it, without executing it?
Suppose my_library were an invalid import. Would Python necessarily raise an ImportError before executing any other commands?
Suppose my_library were a valid module, but it has no function do_things. At what point would Python realize this, during execution of foo() or before?
If anyone could point me to documentation on how Python parses and executes scripts it would be very much appreciated.
There's some information in the tutorial's section on modules, but I don't think the documentation has a complete reference for this. So, here's what happens.
When you first run a script or import a module, Python parses the syntax into an AST and then compiles that into bytecode. It hasn't executed anything yet; it's just compiled your code into instructions for a little stack-based machine. This is where syntax errors are caught. (You can see the guts of all this in the ast module, the token module, the compile builtin, the grammar reference, and sprinkled around various other places.)
You can actually compile a module independently of running the generated code; that's what the builtin compileall method does.
So that's the first phase: compiling. Python only has one other phase, which is actually running the code. Every statement in your module, except those contained within def or lambda, is executed in order. That means that imports happen at runtime, wherever you happen to put them in your module. Which is part of the reason it's good hygiene to put them all at the top. Same for def and class: these are just statements that create a specific type of object, and they're executed as they're encountered, like anything else.
The only tricky bit here is that the phases can happen more than once — for example, an import is only executed at runtime, but if you've never imported that module before, then it has to be compiled, and now you're back in compile time. But "outside" the import it's still runtime, which is why you can catch a SyntaxError thrown by an import.
Anyway, to answer your specific questions:
At compile time. When you run this as a script, or when you import it as a module, or when you compile it with compileall, or otherwise ask Python to make any sense of it. In practical terms, this can happen at any time: if you tried to import this module within a function, you'd only get a SyntaxError when calling that function, which might be halfway through your program.
During the execution of foo, because def and class just create a new object and assign it to a name. But Python still knows how to create the nested function, because it's already compiled all the code within it.
The same way it would add foo = lambda: 1 + 2 to a namespace without executing it. A function is just an object that contains a "code" attribute — literally just a block of Python bytecode. You can manipulate the code type as data, because it is data, independently of executing it. Try looking at a function's .__code__, read the "code objects" section of the data model, or even play around with the disassembler. (You can even execute a code object directly with custom locals and globals using exec, or change the code object a function uses!)
Yes, because import is a plain old statement like any other, executed in order. But if there were other code before the import, that would run first. And if it were in a function, you wouldn't get an error until that function ran. Note that import, just like def and class, is just a fancy form of assignment.
Only during the execution of foo(). Python has no way of knowing whether other code will add a do_things to your module before that point, or even change my_library to some other object entirely. Attribute lookups are always done just-in-time, when you ask for them, never in advance.
As a general rule, python first parses the file, compiles the abstract syntax tree to byte code, then attempt to execute it sequentially. That means all statements are executed line by line. Thus, this means:
Syntax errors are caught at parse time, before anything is executed. If you add some side effect to the script, e.g. create a file, you will see that it never gets executed.
A function becomes defined in the scope after the definition. If you try to call nested_foo right before def nested_foo() you will see that it would fail because nested_foo has not been defined at that point.
Same as 2.
If python cannot import a library, where import means it tries to execute the module, then it fails with an ImportError.
Since you don't try to access do_things at import time (i.e. you are not doing from my_library import do_things), an error only occurs when you attempt to call foo().
$ cat declare_funcs.py
#!/usr/bin/python3
def declared_after():
print("good declared after")
declared_after()
$ python3 declare_funcs.py
good declared after
Change call place:
$ cat declare_funcs.py
#!/usr/bin/python3
declared_after()
def declared_after():
print("good declared after")
$ python3 declare_funcs.py
Traceback (most recent call last):
File "declare_funcs.py", line 4, in <module>
declared_after()
NameError: name 'declared_after' is not defined
Is there way to declare only header of function like it was in C/C++?
For example:
#!/usr/bin/python3
def declared_after() # declaration about defined function
declared_after()
def declared_after():
print("good declared after")
I found this Declare function at end of file in Python
Any way there appear another function in the beginning like wrapper, and this wrapper must be called after declaration of wrapped function, this is not an exit. Is there more elegant true-python way?
You can't forward-declare functions in Python. It doesn't make a lot of sense to do so, because Python is dynamically typed. You could do something silly like this, and what would expect it to do?
foo = 3
foo()
def foo():
print "bar"
Obviously, you are trying to __call__ the int object for 3. It's absolutely silly.
You ask if you can forward-declare like in C/C++. Well, you typically don't run C through an interpreter. However, although Python is compiled to bytecode, the python3 program is an interpreter.
Forward declaration in a compiled language makes sense because you are simply establishing a symbol and its type, and the compiler can run through the code several times to make sense of it. When you use an interpreter, however, you typically can't have that luxury, because you would have to run through the rest of the code to find the meaning of that forward declaration, and run through it again after having done that.
You can, of course, do something like this:
foo = lambda: None
foo()
def foo():
print "bar"
But you instantiated foo nonetheless. Everything has to point to an actual, existing object in Python.
This doesn't apply to def or class statements, though. These create a function or class object, but they don't execute the code inside yet. So, you have time to instantiate things inside them before their code runs.
def foo():
print bar()
# calling foo() won't work yet because you haven't defined bar()
def bar():
return "bar"
# now it will work
The difference was that you simply created function objects with the variable names foo and bar representing them respectively. You can now refer to these objects by those variable names.
With regard to the way that Python is typically interpreted (in CPython) you should make sure that you execute no code in your modules unless they are being run as the main program or unless you want them to do something when being imported (a rare, but valid case). You should do the following:
Put code meant to be executed into function and class definitions.
Unless the code only makes sense to be executed in the main program, put it in another module.
Use if __name__ == "__main__": to create a block of code which will only execute if the program is the main program.
In fact, you should do the third in all of your modules. You can simply write this at the bottom of every file which you don't want to be run as a main program:
if __name__ = "__main__":
pass
This prevents anything from happening if the module is imported.
Python doesn't work that way. The def is executed in sequence, top-to-bottom, with the remainder of the file's contents. You cannot call something before it is defined as a callable (e.g. a function), and even if you had a stand-in callable, it would not contain the code you are looking for.
This, of course, doesn't mean the code isn't compiled before execution begins—in fact, it is. But it is when the def is executed that declared_after is actually assigned the code within the def block, and not before.
Any tricks you pull to sort-of achieve your desired effect must have the effect of delaying the call to declared_after() until after it is defined, for example, by enclosing it in another def block that is itself called later.
One thing you can do is enclose everything in a main function:
def main():
declared_after()
def declared_after():
print("good declared after")
main()
However, the point still stands that the function must be defined prior to calling. This only works because main is called AFTER declared_after is defined.
As zigg wrote, Python files are executed in order they are written from top to bottom, so even if you could “declare” the variable before, the actual function body would only get there after the function was called.
The usual way to solve this is to just have a main function where all your standard execution stuff happens:
def main ():
# do stuff
declared_after();
def declared_after():
pass
main()
You can then also combine this with the __name__ == '__main__' idiom to make the function only execute when you are executing the module directly:
def main ():
# do stuff
declared_after();
def declared_after():
pass
if __name__ == '__main__':
main()
This question already has answers here:
How do I forward-declare a function to avoid `NameError`s for functions defined later?
(17 answers)
Closed 8 months ago.
Is it possible to call a function without first fully defining it? When attempting this I get the error: "function_name is not defined". I am coming from a C++ background so this issue stumps me.
Declaring the function before works:
def Kerma():
return "energy / mass"
print Kerma()
However, attempting to call the function without first defining it gives trouble:
print Kerma()
def Kerma():
return "energy / mass"
In C++, you can declare a function after the call once you place its header before it.
Am I missing something here?
One way that is sort of idiomatic in Python is writing:
def main():
print Kerma()
def Kerma():
return "energy / mass"
if __name__ == '__main__':
main()
This allows you to write you code in the order you like as long as you keep calling the function main at the end.
When a Python module (.py file) is run, the top level statements in it are executed in the order they appear, from top to bottom (beginning to end). This means you can't reference something until you've defined it. For example the following will generate the error shown:
c = a + b # -> NameError: name 'a' is not defined
a = 13
b = 17
Unlike with many other languages, def and class statements are executable in Python—not just declarative—so you can't reference either a or b until that happens and they're defined. This is why your first example has trouble—you're referencing the Kerma() function before its def statement has executed and body have been processed and the resulting function object bound to the function's name, so it's not defined at that point in the script.
Programs in languages like C++ are usually preprocessed before being run and during this compilation stage the entire program and any #include files it refers to are read and processed all at once. Unlike Python, this language features declarative statements which allow the name and calling sequence of functions (or static type of variables) to be declared (but not defined), before use so that when the compiler encounters their name it has enough information to check their usage, which primarily entails type checking and type conversions, none of which requires their actual contents or code bodies to have been defined yet.
This isn't possible in Python, but quite frankly you will soon find you don't need it at all. The Pythonic way to write code is to divide your program into modules that define classes and functions, and a single "main module" that imports all the others and runs.
For simple throw-away scripts get used to placing the "executable portion" at the end, or better yet, learn to use an interactive Python shell.
If you are willing to be like C++ and use everything inside a functions. you can call the first function from the bottom of the file, like this:
def main():
print("I'm in main")
#calling a although it is in the bottom
a()
def b():
print("I'm in b")
def a():
print("I'm in a")
b()
main()
That way python is first 'reading' the whole file and just then starting the execution
Python is a dynamic programming language and the interpreter always takes the state of the variables (functions,...) as they are at the moment of calling them. You could even redefine the functions in some if-blocks and call them each time differently. That's why you have to define them before calling them.