Lambda function in list comprehensions

Question

Why is the output of the following two list comprehensions different  even though f and the lambda function are the same   f   lambda x  x x  f x  for x in range 10     and   lambda x  x x for x in range 10     Mind you  both type f  and type lambda x  x x  return the same type

User · Answer

The big difference is that the first example actually invokes the lambda f(x), while the second example doesn't.

Your first example is equivalent to [(lambda x: x*x)(x) for x in range(10)] while your second example is equivalent to [f for x in range(10)].

User · Answer

The first one   f   lambda x  x x  f x  for x in range 10     runs f   for each value in the range so it does f x  for each value  the second one    lambda x  x x for x in range 10     runs the lambda for each value in the list  so it generates all of those functions

User · Answer

The other answers are correct  but if you are trying to make a list of functions  each with a different parameter  that can be executed later  the following code will do that   import functools a    functools partial lambda x  x x  x  for x in range 10    b      for i in a      b append i     In  26   b Out 26    0  1  4  9  16  25  36  49  64  81    While the example is contrived  I found it useful when I wanted a list of functions that each print something different  i e   import functools a    functools partial lambda x  print x   x  for x in range 10    for i in a      i

User · Answer

People gave good answers but forgot to mention the most important part in my opinion  In the second example the X of the list comprehension is NOT the same as the X of the lambda function  they are totally unrelated  So the second example is actually the same as    Lambda X  X X for I in range 10     The internal iterations on range 10  are only responsible for creating 10 similar lambda functions in a list  10 separate functions but totally similar - returning the power 2 of each input    On the other hand  the first example works totally different  because the X of the iterations DO interact with the results  for each iteration the value is X X so the result would be  0 1 4 9 16 25  36  49  64  81

User · Answer

The first one creates a single lambda function and calls it ten times  The second one doesn t call the function  It creates 10 different lambda functions  It puts all of those in a list  To make it equivalent to the first you need    lambda x  x x  x  for x in range 10    Or better yet   x x for x in range 10

User · Answer

This question touches a very stinking part of the  quot famous quot  and  quot obvious quot  Python syntax - what takes precedence  the lambda  or the for of list comprehension  I don t think the purpose of the OP was to generate a list of squares from 0 to 9  If that was the case  we could give even more solutions  squares      for x in range 10   squares append x x    this is the good ol  way of imperative syntax   But it s not the point  The point is W hy TF is this ambiguous expression so counter-intuitive  And I have an idiotic case for you at the end  so don t dismiss my answer too early  I had it on a job interview   So  the OP s comprehension returned a list of lambdas    lambda x  x x  for x in range 10    This is of course just 10 different copies of the squaring function  see   gt  gt  gt   lambda x  x x for   in range 3     lt function  lt lambda gt  at 0x00000000023AD438 gt    lt function  lt lambda gt  at 0x00000000023AD4A8 gt    lt function  lt lambda gt  at 0x00000000023AD3C8 gt    Note the memory addresses of the lambdas - they are all different  You could of course have a more  quot optimal quot   haha  version of this expression   gt  gt  gt   lambda x  x x    3   lt function  lt lambda gt  at 0x00000000023AD2E8 gt    lt function  lt lambda gt  at 0x00000000023AD2E8 gt    lt function  lt lambda gt  at 0x00000000023AD2E8 gt    See  3 time the same lambda  Please note  that I used   as the for variable  It has nothing to do with the x in the lambda  it is overshadowed lexically    Get it  I m leaving out the discussion  why the syntax precedence is not so  that it all meant   lambda x   x x for x in range 10     which could be    0  1  4       81    or   0  1  4       81    or which I find most logical  this would be a list of 1 element - a generator returning the values  It is just not the case  the language doesn t work this way  BUT What  If    What if you DON T overshadow the for variable  AND use it in your lambdas    Well  then crap happens  Look at this   lambda x  x   i for i in range 4    this means of course    lambda x  x   i  for i in range 4    BUT it DOESN T mean    lambda x  x   0    lambda x  x   1        lambda x  x   3    This is just crazy  The lambdas in the list comprehension are a closure over the scope of this comprehension  A lexical closure  so they refer to the i via reference  and not its value when they were evaluated  So  this expression    lambda x  x   i  for i in range 4    IS roughly EQUIVALENT to    lambda x  x   3    lambda x  x   3        lambda x  x   3    I m sure we could see more here using a python decompiler  by which I mean e g  the dis module   but for Python-VM-agnostic discussion this is enough  So much for the job interview question  Now  how to make a list of multiplier lambdas  which really multiply by consecutive integers  Well  similarly to the accepted answer  we need to break the direct tie to i by wrapping it in another lambda  which is getting called inside the list comprehension expression  Before   gt  gt  gt  a     lambda x  x   i  for i in  1  2    gt  gt  gt  a 1  1  2  gt  gt  gt  a 0  1  2  After   gt  gt  gt  a     lambda y   lambda x  y   x   i  for i in  1  2    gt  gt  gt  a 1  1  2  gt  gt  gt  a 0  1  1   I had the outer lambda variable also   i  but I decided this is the clearer solution - I introduced y so that we can all see which witch is which   Edit 2019-08-30  Following a suggestion by  josoler  which is also present in an answer by  sheridp - the value of the list comprehension  quot loop variable quot  can be  quot embedded quot  inside an object - the key is for it to be accessed at the right time  The section  quot After quot  above does it by wrapping it in another lambda and calling it immediately with the current value of i  Another way  a little bit easier to read - it produces no  WAT  effect  is to store the value of i inside a partial object  and have the  quot inner quot   original  lambda take it as an argument  passed supplied by the partial object at the time of the call   i e   After 2   gt  gt  gt  from functools import partial  gt  gt  gt  a    partial lambda y  x  y   x  i  for i in  1  2    gt  gt  gt  a 0  2   a 1  2   2  4   Great  but there is still a little twist for you  Let s say we wan t to make it easier on the code reader  and pass the factor by name  as a keyword argument to partial   Let s do some renaming  After 2 5   gt  gt  gt  a    partial lambda coef  x  coef   x  coef i  for i in  1  2    gt  gt  gt  a 0  1  Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  TypeError   lt lambda gt    got multiple values for argument  coef   WAT   gt  gt  gt  a 0    Traceback  most recent call last   File  quot  lt stdin gt  quot   line 1  in  lt module gt  TypeError   lt lambda gt    missing 1 required positional argument   x   Wait    We re changing the number of arguments by 1  and going from  quot too many quot  to  quot too few quot   Well  it s not a real WAT  when we pass coef to partial in this way  it becomes a keyword argument  so it must come after the positional x argument  like so  After 3   gt  gt  gt  a    partial lambda x  coef  coef   x  coef i  for i in  1  2    gt  gt  gt  a 0  2   a 1  2   2  4   I would prefer the last version over the nested lambda  but to each their own    Edit 2020-08-18  Thanks to commenter dasWesen  I found out that this stuff is covered in the Python documentation  https   docs python org 3 4 faq programming html why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result - it deals with loops instead of list comprehensions  but the idea is the same - global or nonlocal variable access in the lambda function  There s even a solution - using default argument values  like for any function    gt  gt  gt  a    lambda x  coef i  coef   x for i in  1  2    gt  gt  gt  a 0  2   a 1  2   2  4   This way the coef value is bound to the value of i at the time of function definition  see James Powell s talk  quot Top To Down  Left To Right quot   which also explains why mutable default values are shunned

[python] Lambda function in list comprehensions

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