CODEWITHSUNDEEP
pythonfor-loopreturnlist-comprehension
lailai's leaves
lailai's leaves

Reputation: 5

Python - print and return in for loops

I've just started learning Python. I saw an interesting code from here that the author used to explain short-circuiting. The code is as follows:

>>> def fun(i):
...     print "executed"
...     return i
...

I tried to call fun(1). The output is the following and it makes perfect sense to me.

>>> fun(1)
executed
1

Then I tried [fun(i) for i in [0, 1, 2, 3]], and the output looked like this:

>>> [fun(i) for i in [0, 1, 2, 3]]
executed
executed
executed
executed
[0, 1, 2, 3]

I was expecting something like this:

executed
0
executed
1
executed
2
executed
3

Can anyone tell me what I got wrong? Thank you!

Upvotes: 0

Views: 165

Answers (3)

Daweo
Daweo

Reputation: 36360

>>> def fun(i):
...     print "executed"
...     return i
...

This is not example of short-circuiting. This is function with side effect - it does write to standard output, which is observable effect besides returning a value.

[fun(i) for i in [0, 1, 2, 3]]

This is list comprehension. Generally speaking you should have good reason to use function with side effect in such place, PEP 202 explains motivation for list comprehension as follows:

List comprehensions provide a more concise way to create lists in situations where map() and filter() and/or nested loops would currently be used.

Someone reading your code therefore will probably assume it has to just create list, without printing elements during this action.

Upvotes: 3

user2952903
user2952903

Reputation: 385

Square brakets in this context form a new list, which each element is function fun of according element in [0, 1, 2, 3]. It is called list list-comprehensions. "executed" is printed because during formation of new list function fun is called(for each element in [0, 1, 2, 3]). numbers 0,1,2 and 3 are not printed, because return value of fun is not printed, but put in the new list. To see the new list:

print("new list:",[fun(i) for i in [0, 1, 2, 3]])

Upvotes: 1

Be Chiller Too
Be Chiller Too

Reputation: 2900

Try doing

l = [fun(i) for i in [0, 1, 2, 3]]

This will output

executed
executed
executed
executed

Then just execute l, this will display the value of l, that is to say: [0, 1, 2, 3]

So when you are executing

>>> [fun(i) for i in [0, 1, 2, 3]]

This calls fun(0), fun(1), and so on, and displays executed, then it displays the computed value: [0, 1, 2, 3]

Upvotes: 0

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