Question

Does map() iterate through the list like “for” would? Is there a value in using map vs for?

If so, right now my code looks like this:

for item in items:
    item.my_func()

If it makes sense, I would like to make it map(). Is that possible? What is an example like?

Answer 1

You could use map instead of the for loop you’ve shown, but since you do not appear to use the result of item.my_func(), this is not recommended. map should be used if you want to apply a function without side-effects to all elements of a list. In all other situations, use an explicit for-loop.

Also, as of Python 3.0 map returns an iterator, so in that case map will not behave the same (unless you explicitly evaluate all elements returned by the iterator, e.g. by calling list on it).

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Edit: kibibu asks in the comments for a clarification on why map‘s first argument should not be a function with side effects. I’ll give answering that question a shot:

map is meant to be passed a function f in the mathematical sense. Under such circumstances it does not matter in which order f is applied to the elements of the second argument (as long as they are returned in their original order, of course). More importantly, under those circumstances map(g, map(f, l)) is semantically equivalent to map(lambda x: g(f(x)), l), regardless of the order in which f and g are applied to their respective inputs.

E.g., it doesn’t matter whether map returns an iterator or a full list at once. However, if f and/or g cause side effects, then this equivalence is only guaranteed if the semantics of map(g, map(f, l)) are such that at any stage g is applied to the first n elements returned by map(f, l) before map(f, l) applies f to the *(n + 1)*​st element of l. (Meaning that map must perform the laziest possible iteration—which it does in Python 3, but not in Python 2!)

Going one step further: even if we assume the Python 3 implementation of map, the semantic equivalence may easily break down if the output of map(f, l) is e.g. passed through itertools.tee before being supplied to the outer map call.

The above discussion may seem of a theoretic nature, but as programs become more complex, they become more difficult to reason about and therefore harder to debug. Ensuring that some things are invariant alleviates that problem somewhat, and may in fact prevent a whole class of bugs.

Lastly, map reminds many people of its truly functional counterpart in various (purely) functional languages. Passing it a "function" with side effects will confuse those people. Therefore, seeing as the alternative (i.e., using an explicit loop) is not harder to implement than a call to map, it is highly recommended that one restricts use of map to those cases in which the function to be applied does not cause side effects.