Weird lambda behaviour in loops

Question

I stumbled upon a behaviour in python that I have a hard time understanding. This is the proof-of-concept code:

from functools import partial

if __name__ == '__main__':
    sequence = ['foo', 'bar', 'spam']
    loop_one = lambda seq: [lambda: el for el in seq]
    no_op = lambda x: x
    loop_two = lambda seq: [partial(no_op, el) for el in seq]
    for func in (loop_one, loop_two):
        print [f() for f in func(sequence)]

The output of the above is:

['spam', 'spam', 'spam']
['foo', 'bar', 'spam']

The behaviour of loop_one is surprising to me as I would expect it to behave as loop_two:el is an immutable value (a string) that changes at each loop, but lambda seems to store a pointer to the “looping variable”, like if the loop would recycle the same memory address for each element of the sequence.

The above behaviour is the same with full-blown functions with a for loop in them (so it is not a list-comprehension syntax).

But wait: there is more… and more puzzling!

The following script works like loop_one:

b = []
for foo in ("foo", "bar"):
    b.append(lambda: foo)

print [a() for a in b]

(output: ['bar', 'bar'])

But watch what happens when one substitute the variable name foo with a:

b = []
for a in ("foo", "bar"):
    b.append(lambda: a)

print [a() for a in b]

(output: [<function <lambda> at 0x25cce60>, <function <lambda> at 0x25cced8>])

Any idea of what is happening here? I suspect there must be some gotcha related to the underlying C implementation of my interpreter, but I haven’t anything else (Jthon, PyPy or similar) to test if this behaviour is consistent across different implementations.

Asked By: mac

||

Source

Answer 1

The variables (foo in the following example) is binded not when the lambda is created, but when the lambda is called.

>>> b = []
>>> for foo in ("foo", "bar"):
...     b.append(lambda: foo)
...
>>> foo = "spam"
>>> print [a() for a in b]
['spam', 'spam']

>>> b = []
>>> for foo in ("foo", "bar"):
...     b.append(lambda foo=foo: foo)
...
>>> print [a() for a in b]
['foo', 'bar']

Answered By: falsetru

Answer 2

The function lambda: el used in loop_one refers to a variable el which is not defined in the local scope. Therefore, Python looks for it next in the enclosing scope of the other lambda:

lambda seq: [lambda: el for el in seq]

in accordance with the so-called LEGB rule.

By the time lambda: el is called, this enclosing lambda has (of course) already been called and the list comprehension has been evaluated. The el used in the list comprehension is a local variable in this enclosing lambda. Its value is the one returned when Python looks for the value of el in lambda: el. That value for el is the same for all the different lambda: el functions in the list comprehension: it is the last value assigned to el in the for el in seq loop. Thus, el is always 'spam', the last value in seq.

You’ve already found one workaround, to use a closure such as your loop_two. Another way is to define el as a local variable with a default value:

loop_one = lambda seq: [lambda el=el: el for el in seq]

Answered By: unutbu

Weird lambda behaviour in loops

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