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This question already has answers here:
What is `lambda` in Python code? How does it work with `key` arguments to `sorted`, `sum` etc.?
(4 answers)
Closed 6 months ago.
I want to know what exactly is lambda in python? and where and why it is used.
thanks
Lambda is more of a concept or programming technique then anything else.
Basically it's the idea that you get a function (a first-class object in python) returned as a result of another function instead of an object or primitive type. I know, it's confusing.
See this example from the python documentation:
def make_incrementor(n):
return lambda x: x + n
f = make_incrementor(42)
f(0)
>>> 42
f(1)
>>> 43
So make_incrementor creates a function that uses n in it's results. You could have a function that would increment a parameter by 2 like so:
f2 = make_incrementor(2)
f2(3)
>>> 5
This is a very powerful idea in functional programming and functional programming languages like lisp & scheme.
Hope this helps.
Lambdas are not anonymous functions. Lambdas are anonymous expressions.
They're accessed like functions, but they're not the same thing. Functions allow complex tasks: flow control, variable declarations and lists of statements containing expressions. Expressions are merely one part of a function, and that's what lambdas give you. They're severely limited compared to functions.
Python does not support anonymous functions. For examples of languages that do, see Javascript and Lua.
(Note: It's correct to call lambdas anonymous functions in functional languages, where the mathematical definition of "function" is used, but in procedural languages the word has a very different meaning than in mathematics.)
Lambda functions are not a Python specific concept, but are a general programming term for anonymous function, i.e. functions without a name. In Python they are commonly used where you need to pass a simple function as a parameter to another function.
The sort method on lists takes a parameter key which is a function that is used to calculate the value the list is sorted on. Imagine you are sorting a list of two element tuples, and you want to sort the list based on the first element. You need to pass a function to key which returns the first element. You could do this:
def first_element(x):
return x[0]
my_list.sort(key=first_element)
or, much more concisely you can do:
my_list.sort(key=lambda x: x[0])
The lambda construct is a shorter way to define a simple function that calculates a single expression. The def statement can be inconvenient and make the code longer, broken up and harder to read through. The functions created by the two are virtually the same by the way they work, the difference is that lambda is limited to a single expression the value of which is returned and that def assigns a name to the function and adds it to the local variables by that same name. Also lambda can be used in an expression directly, while def is a statement.
def f(x, y):
return x + y
Would give you almost the same result as
f = lambda x, y: x + y
And you can use it directly in an expression
g(5, 6, helper=lambda x, y: x + y)
which with def would be less concise
def helper_function(x + y):
return x + y
g(5, 6, helper=helper_function)
lambda is a way of defining an anonymous in-line function to accomplish some task versus defining it with the def keyword and calling it. Think of it as a shorthand you can use when you won't need to reuse a function. Anywhere you use lambda, you could substitute an explicitly called function: it's use is completely optional and left to the programmer as a matter of coding style.
Example code (without lambda):
def numSquared(num):
return num**2
for i in range(1,11):
print numSquared(i)
Example code (with lambda):
for i in range(1,11):
print (lambda x: x**2)(i)
A good beginner's discussion of lambda:
DiveIntPython.org - Lambda
lambda allows you define simple, unnamed functions inline with your code, e.g. as an argument. This is useful if you plan to use the function only once, and therefore don't want to clutter your code with a named function.
Let's say you have a list of numbers (thelist), and you want to get a list of the same length, with each number doubled. Rather than defining a times_two function, you could do something like this:
map(lambda x: x * 2, thelist)
lambda also makes Currying more convenient.
The first linked returned by googling "python lambda" points to a page whose first paragraphs answer your question. I could copy & paste the content it, but I think it's unfair :)
It's an inline anonymous function.
lambda is an anonymous function, usually used for something quick that needs computing.
Example (This is used in a LexYacc command parser):
assign_command = lambda dictionary, command: lambda command_function: dictionary.setdefault(command, command_function)
This is a decorator. Put before a command we'd have from the LexYacc parser:
#assign_command(_command_handlers, 'COMMAND')
This essentially builds a python script from the LexYacc language defined.
Or, a bit simpler:
`x = lambda x: return x > 0 and x & (x-1) == 0
Lambda can be interpreted intuitively as algebra function.
Suppose a basic algebra y = x**2 + 1
if typed directly,
>>> y = x**2 + 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
error occurs and reports that variable 'x' should be defined firstly.
nevertheless,'x' is unknown, how can it be pre-defined.
Lambda helps:
python
>>> y = lambda x: x**2 + 1 # no errors reports
>>> y(3)
10
That's core about lambda,
It's in programming language to write an inline function.
Lambda is an anonymous function in Python programming language, instead of bearing a def statement in the front it is simply called and written lambda.
def mult2(x):
return x*2
print(mult2(2))
# returns 4
Exactly same you can do with a lambda function.
lam = lambda x : x*2
print(lam(2))
# returns 4
For more details you can look up in this blog post.
http://friendlypython.herokuapp.com/2017/1/anonymous-function-lambda/ or here
I would like to explain this with a little layman approach in terms of the usage perspective, since all the technical points are already covered.
A lambda function is just like any other function, only thing that makes it different (other than the syntax and the fact that it can't be reused), is that it is used when we want to quickly write a function which takes an argument and just returns a value using only one expression.
For instance:
If all that a function does is take a argument and add 1 to it then using a lambda function is a better approach than a normal function.
But if your function requires more than one line of processing before returning the value, then we need to use a normal function definition and then call it.
I'm beginning to appreciate the value of lambda expressions in python, particularly when it comes to functional programming, map, functions returning functions, etc. However, I've also been naming lambdas within functions because:
I need the same functionality several times and don't want to repeat code.
The functionality is specific to the function in which it appears; its not needed elsewhere.
When I encounter a situation that meets the above criteria, I've been writing a named lambda expression in order to DRY and narrowly scope functionality. For example, I am writing a function that operates on some numpy arrays, and I need to do some moderately tedious indexing of all the arrays passed to the function (which can easily fit on a single line). I've written a named lambda expression to do the indexing instead of writing a whole other function or copy/pasting the indexing several times throughout the function definition.
def fcn_operating_on_arrays(array0, array1):
indexer = lambda a0, a1, idx: a0[idx] + a1[idx]
# codecodecode
indexed = indexer(array0, array1, indices)
# codecodecode in which other arrays are created and require `indexer`
return the_answer
Is this an abuse of python's lambdas? Should I just suck it up and define a separate function?
Edits
Probably worth linking function inside function.
This is not Pythonic and PEP8 discourages it:
Always use a def statement instead of an assignment statement that
binds a lambda expression directly to an identifier.
Yes:
def f(x): return 2*x
No:
f = lambda x: 2*x
The first form means that the name of the resulting function object is
specifically 'f' instead of the generic '<lambda>'. This is more
useful for tracebacks and string representations in general. The use
of the assignment statement eliminates the sole benefit a lambda
expression can offer over an explicit def statement (i.e. that it can
be embedded inside a larger expression)
A rule of thumb for this is to think on its definition: lambdas expressions are anonymous functions. If you name it, it isn't anonymous anymore. :)
I've written a named lambda expression to do the indexing instead of writing a whole other function
Well, you are writing a whole other function. You're just writing it with a lambda expression.
Why not use def? You get nicer stack traces and more syntactical flexibility, and you don't lose anything. It's not like def can't occur inside another function:
def fcn_operating_on_arrays(array0, array1):
def indexer(a0, a1, idx):
return a0[idx] + a1[idx]
...
This question already has answers here:
What is `lambda` in Python code? How does it work with `key` arguments to `sorted`, `sum` etc.?
(4 answers)
Closed 6 months ago.
I want to know what exactly is lambda in python? and where and why it is used.
thanks
Lambda is more of a concept or programming technique then anything else.
Basically it's the idea that you get a function (a first-class object in python) returned as a result of another function instead of an object or primitive type. I know, it's confusing.
See this example from the python documentation:
def make_incrementor(n):
return lambda x: x + n
f = make_incrementor(42)
f(0)
>>> 42
f(1)
>>> 43
So make_incrementor creates a function that uses n in it's results. You could have a function that would increment a parameter by 2 like so:
f2 = make_incrementor(2)
f2(3)
>>> 5
This is a very powerful idea in functional programming and functional programming languages like lisp & scheme.
Hope this helps.
Lambdas are not anonymous functions. Lambdas are anonymous expressions.
They're accessed like functions, but they're not the same thing. Functions allow complex tasks: flow control, variable declarations and lists of statements containing expressions. Expressions are merely one part of a function, and that's what lambdas give you. They're severely limited compared to functions.
Python does not support anonymous functions. For examples of languages that do, see Javascript and Lua.
(Note: It's correct to call lambdas anonymous functions in functional languages, where the mathematical definition of "function" is used, but in procedural languages the word has a very different meaning than in mathematics.)
Lambda functions are not a Python specific concept, but are a general programming term for anonymous function, i.e. functions without a name. In Python they are commonly used where you need to pass a simple function as a parameter to another function.
The sort method on lists takes a parameter key which is a function that is used to calculate the value the list is sorted on. Imagine you are sorting a list of two element tuples, and you want to sort the list based on the first element. You need to pass a function to key which returns the first element. You could do this:
def first_element(x):
return x[0]
my_list.sort(key=first_element)
or, much more concisely you can do:
my_list.sort(key=lambda x: x[0])
The lambda construct is a shorter way to define a simple function that calculates a single expression. The def statement can be inconvenient and make the code longer, broken up and harder to read through. The functions created by the two are virtually the same by the way they work, the difference is that lambda is limited to a single expression the value of which is returned and that def assigns a name to the function and adds it to the local variables by that same name. Also lambda can be used in an expression directly, while def is a statement.
def f(x, y):
return x + y
Would give you almost the same result as
f = lambda x, y: x + y
And you can use it directly in an expression
g(5, 6, helper=lambda x, y: x + y)
which with def would be less concise
def helper_function(x + y):
return x + y
g(5, 6, helper=helper_function)
lambda is a way of defining an anonymous in-line function to accomplish some task versus defining it with the def keyword and calling it. Think of it as a shorthand you can use when you won't need to reuse a function. Anywhere you use lambda, you could substitute an explicitly called function: it's use is completely optional and left to the programmer as a matter of coding style.
Example code (without lambda):
def numSquared(num):
return num**2
for i in range(1,11):
print numSquared(i)
Example code (with lambda):
for i in range(1,11):
print (lambda x: x**2)(i)
A good beginner's discussion of lambda:
DiveIntPython.org - Lambda
lambda allows you define simple, unnamed functions inline with your code, e.g. as an argument. This is useful if you plan to use the function only once, and therefore don't want to clutter your code with a named function.
Let's say you have a list of numbers (thelist), and you want to get a list of the same length, with each number doubled. Rather than defining a times_two function, you could do something like this:
map(lambda x: x * 2, thelist)
lambda also makes Currying more convenient.
The first linked returned by googling "python lambda" points to a page whose first paragraphs answer your question. I could copy & paste the content it, but I think it's unfair :)
It's an inline anonymous function.
lambda is an anonymous function, usually used for something quick that needs computing.
Example (This is used in a LexYacc command parser):
assign_command = lambda dictionary, command: lambda command_function: dictionary.setdefault(command, command_function)
This is a decorator. Put before a command we'd have from the LexYacc parser:
#assign_command(_command_handlers, 'COMMAND')
This essentially builds a python script from the LexYacc language defined.
Or, a bit simpler:
`x = lambda x: return x > 0 and x & (x-1) == 0
Lambda can be interpreted intuitively as algebra function.
Suppose a basic algebra y = x**2 + 1
if typed directly,
>>> y = x**2 + 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
error occurs and reports that variable 'x' should be defined firstly.
nevertheless,'x' is unknown, how can it be pre-defined.
Lambda helps:
python
>>> y = lambda x: x**2 + 1 # no errors reports
>>> y(3)
10
That's core about lambda,
It's in programming language to write an inline function.
Lambda is an anonymous function in Python programming language, instead of bearing a def statement in the front it is simply called and written lambda.
def mult2(x):
return x*2
print(mult2(2))
# returns 4
Exactly same you can do with a lambda function.
lam = lambda x : x*2
print(lam(2))
# returns 4
For more details you can look up in this blog post.
http://friendlypython.herokuapp.com/2017/1/anonymous-function-lambda/ or here
I would like to explain this with a little layman approach in terms of the usage perspective, since all the technical points are already covered.
A lambda function is just like any other function, only thing that makes it different (other than the syntax and the fact that it can't be reused), is that it is used when we want to quickly write a function which takes an argument and just returns a value using only one expression.
For instance:
If all that a function does is take a argument and add 1 to it then using a lambda function is a better approach than a normal function.
But if your function requires more than one line of processing before returning the value, then we need to use a normal function definition and then call it.
Does the Python language have a built-in function for an analog of map that sends an argument to a sequence of functions, rather than a function to a sequence of arguments?
Plain map would have "type" (thinking like Haskell) (a -> b) -> [a] -> [b]; is there anything with the corresponding type a -> [(a -> b)] -> [b]?
I could implement this in a number of ways. Here's using a lambda
def rev_map(x, seq):
evaluate_yourself_at_x = lambda f: f(x)
return map(evaluate_yourself_at_x, seq)
rev_map([1,2], [sum, len, type])
which prints [3, 2, list].
I'm just curious if this concept of "induce a function to evaluate itself at me" has a built-in or commonly used form.
One motivation for me is thinking about dual spaces in functional analysis, where a space of elements which used to be conceived of as arguments passed to functions is suddenly conceived of as a space of elements which are functions whose operation is to induce another function to be evaluated at them.
You could think of a function like sin as being an infinite map from numbers to numbers, you give sin a number, sin gives you some associated number back, like sin(3) or something.
But then you could also think of the number 3 as an infinite map from functions to numbers, you give 3 a function f and 3 gives you some associated number, namely f(3).
I'm finding cases where I'd like some efficient syntax to suddenly view "arguments" or "elements" as "function-call-inducers" but most things, e.g. my lambda approach above, seem clunky.
Another thought I had was to write wrapper classes for the "elements" where this occurs. Something like:
from __future__ import print_function
class MyFloat(float):
def __call__(self, f):
return f(self)
m = MyFloat(3)
n = MyFloat(2)
MyFloat(m + n)(type)
MyFloat(m + n)(print)
which will print __main__.MyFloat and 5.0.
But this requires a lot of overhead to redefine data model operators and so on, and clearly it's not a good idea to push around your own version of very basic things like float which will be ubiquitous in most programs. It's also easy to get it wrong, like from my example above, doing this:
# Will result in a recursion error.
MyFloat(3)(MyFloat(4))
There is no built-in function for that. Simply because that's definitely not a commonly used concept. Plus Python is not designed to solve mathematical problems.
As for the implementation here's the shortest one you can get IMHO:
rev_map = lambda x, seq: [f(x) for f in seq]
Note that the list comprehension is so short and easy that wrapping it with a function seems to be unnecessary in the first place.
This question already has answers here:
What is `lambda` in Python code? How does it work with `key` arguments to `sorted`, `sum` etc.?
(4 answers)
Closed 6 months ago.
I want to know what exactly is lambda in python? and where and why it is used.
thanks
Lambda is more of a concept or programming technique then anything else.
Basically it's the idea that you get a function (a first-class object in python) returned as a result of another function instead of an object or primitive type. I know, it's confusing.
See this example from the python documentation:
def make_incrementor(n):
return lambda x: x + n
f = make_incrementor(42)
f(0)
>>> 42
f(1)
>>> 43
So make_incrementor creates a function that uses n in it's results. You could have a function that would increment a parameter by 2 like so:
f2 = make_incrementor(2)
f2(3)
>>> 5
This is a very powerful idea in functional programming and functional programming languages like lisp & scheme.
Hope this helps.
Lambdas are not anonymous functions. Lambdas are anonymous expressions.
They're accessed like functions, but they're not the same thing. Functions allow complex tasks: flow control, variable declarations and lists of statements containing expressions. Expressions are merely one part of a function, and that's what lambdas give you. They're severely limited compared to functions.
Python does not support anonymous functions. For examples of languages that do, see Javascript and Lua.
(Note: It's correct to call lambdas anonymous functions in functional languages, where the mathematical definition of "function" is used, but in procedural languages the word has a very different meaning than in mathematics.)
Lambda functions are not a Python specific concept, but are a general programming term for anonymous function, i.e. functions without a name. In Python they are commonly used where you need to pass a simple function as a parameter to another function.
The sort method on lists takes a parameter key which is a function that is used to calculate the value the list is sorted on. Imagine you are sorting a list of two element tuples, and you want to sort the list based on the first element. You need to pass a function to key which returns the first element. You could do this:
def first_element(x):
return x[0]
my_list.sort(key=first_element)
or, much more concisely you can do:
my_list.sort(key=lambda x: x[0])
The lambda construct is a shorter way to define a simple function that calculates a single expression. The def statement can be inconvenient and make the code longer, broken up and harder to read through. The functions created by the two are virtually the same by the way they work, the difference is that lambda is limited to a single expression the value of which is returned and that def assigns a name to the function and adds it to the local variables by that same name. Also lambda can be used in an expression directly, while def is a statement.
def f(x, y):
return x + y
Would give you almost the same result as
f = lambda x, y: x + y
And you can use it directly in an expression
g(5, 6, helper=lambda x, y: x + y)
which with def would be less concise
def helper_function(x + y):
return x + y
g(5, 6, helper=helper_function)
lambda is a way of defining an anonymous in-line function to accomplish some task versus defining it with the def keyword and calling it. Think of it as a shorthand you can use when you won't need to reuse a function. Anywhere you use lambda, you could substitute an explicitly called function: it's use is completely optional and left to the programmer as a matter of coding style.
Example code (without lambda):
def numSquared(num):
return num**2
for i in range(1,11):
print numSquared(i)
Example code (with lambda):
for i in range(1,11):
print (lambda x: x**2)(i)
A good beginner's discussion of lambda:
DiveIntPython.org - Lambda
lambda allows you define simple, unnamed functions inline with your code, e.g. as an argument. This is useful if you plan to use the function only once, and therefore don't want to clutter your code with a named function.
Let's say you have a list of numbers (thelist), and you want to get a list of the same length, with each number doubled. Rather than defining a times_two function, you could do something like this:
map(lambda x: x * 2, thelist)
lambda also makes Currying more convenient.
The first linked returned by googling "python lambda" points to a page whose first paragraphs answer your question. I could copy & paste the content it, but I think it's unfair :)
It's an inline anonymous function.
lambda is an anonymous function, usually used for something quick that needs computing.
Example (This is used in a LexYacc command parser):
assign_command = lambda dictionary, command: lambda command_function: dictionary.setdefault(command, command_function)
This is a decorator. Put before a command we'd have from the LexYacc parser:
#assign_command(_command_handlers, 'COMMAND')
This essentially builds a python script from the LexYacc language defined.
Or, a bit simpler:
`x = lambda x: return x > 0 and x & (x-1) == 0
Lambda can be interpreted intuitively as algebra function.
Suppose a basic algebra y = x**2 + 1
if typed directly,
>>> y = x**2 + 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined
error occurs and reports that variable 'x' should be defined firstly.
nevertheless,'x' is unknown, how can it be pre-defined.
Lambda helps:
python
>>> y = lambda x: x**2 + 1 # no errors reports
>>> y(3)
10
That's core about lambda,
It's in programming language to write an inline function.
Lambda is an anonymous function in Python programming language, instead of bearing a def statement in the front it is simply called and written lambda.
def mult2(x):
return x*2
print(mult2(2))
# returns 4
Exactly same you can do with a lambda function.
lam = lambda x : x*2
print(lam(2))
# returns 4
For more details you can look up in this blog post.
http://friendlypython.herokuapp.com/2017/1/anonymous-function-lambda/ or here
I would like to explain this with a little layman approach in terms of the usage perspective, since all the technical points are already covered.
A lambda function is just like any other function, only thing that makes it different (other than the syntax and the fact that it can't be reused), is that it is used when we want to quickly write a function which takes an argument and just returns a value using only one expression.
For instance:
If all that a function does is take a argument and add 1 to it then using a lambda function is a better approach than a normal function.
But if your function requires more than one line of processing before returning the value, then we need to use a normal function definition and then call it.