Python map function, passing by reference/value?

Question

I have a question about the map function in Python.

From what I understand, the function does not mutate the list it\'s operating on, but rather create

Answer 1

I want to bump this. The answers don't really answer the question - they disregard the fact that the OP was able to achieve the desired result by iterating. The question comes down to the behavior of map. Here is a more direct example:

f=(lambda pair: pair[0].append(pair[1]))
a = ([],1)
f(a)
print(a) #prints ([1],1)
a=([],1)
map(f,[a])
print(a) #prints ([0],1)

So map isn't mutating objects in the way the OP is expecting. I have the same confusion.

Can anyone comment on exactly what is going on here? I think that'd be a good answer to the OP's question.

Note that we have different behavior if we assign the output of map as follows (as per Cat Plus Plus' answer)

f=(lambda pair: pair[0].append(pair[1]))
a = ([],1)
x = [a]
x[:] = map(f,x)
print(x) #prints None
print(a) # prints [1]

Please note that in the first example, we simply called f(a), not a=f(a). Why do we need assignment when using map and not when working outside of map?