calling a function from a list in python

Question

Take this example:

presets = [
    "eggs",
    "bacon"
    ]

print(presets[0])
>>> eggs

Why can a not do the same thing, with a list of items to execute? Take this example:

from animations import animation_2, animation_3, animation_4
presets = [
    animation_2.iterate(animations_templates_path, thumbnails_final),
    animation_3.iterate(animations_templates_path, thumbnails_final),
    animation_4.iterate(animations_templates_path, thumbnails_final)
    ]

When I run this (both WITH and WITHOUT the preset[n]) it executes all three commands in the list. Why is this? I would like to have a list of those presets, and call them via am index number. What am I doing wrong?

You can do exactly the same thing, if you store the callables and not the result of calling them. Just store e.g. animation_2.iterate, then pass the arguments when you take them back out of the list. — jonrsharpe
– jonrsharpe, Commented Dec 21, 2016 at 17:17
Store as presets = [animation_2.interate, ...]. Then call by .............................................. presets[0](animations_templates_path, thumbnails_final) — pylang
– pylang, Commented Dec 21, 2016 at 17:20
If you saw [factorial(4), factorial(5)] in a program, would you think the first element of that list is a command you can retrieve and execute later, or would you think it's the factorial of 4? Same with your code. — user2357112
– user2357112, Commented Dec 21, 2016 at 17:21

Bryan Oakley · Accepted Answer · 2016-12-21 17:19:21Z

2

It executes the items because that's what you are telling it to do. Your code is exactly the same as this:

p1 = animation_2.iterate(animations_templates_path, thumbnails_final)
p2 = animation_3.iterate(animations_templates_path, thumbnails_final)
p3 = animation_4.iterate(animations_templates_path, thumbnails_final)
presets = [p1, p2, p3]

Python has no way of knowing that you didn't intend to call those functions.

One solution is to store a tuple:

presets = [
    (animation_2.iterate, animations_templates_path, thumbnails_final),
    (animation_3.iterate, animations_templates_path, thumbnails_final),
    (animation_4.iterate(animations_templates_path, thumbnails_final),

]

That stores the function and the arguments without calling the function. You can iterate over the list at a later date and execute the function.

answered Dec 21, 2016 at 17:19

Bryan Oakley

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1 Comment

M Leonard Over a year ago

This is extremely helpful in allowing me to understand what I am doing. However, I am going to mark the other answer as the solution because in my use case, it allow me to write less code and iterate over it easier.

Cory Kramer · Accepted Answer · 2016-12-21 17:19:37Z

1

You can store the actual function objects in a list

from animations import animation_1, animation_2, animation_3
presets = [
    animation_2.iterate,
    animation_3.iterate,
    animation_4.iterate
    ]

Then call the desired function based on its index. This way the function is not executed upon constructing the list, rather it is only executed once you call it.

presets[0](animations_templates_path, thumbnails_final)

answered Dec 21, 2016 at 17:19

Cory Kramer

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1 Comment

M Leonard Over a year ago

This is exactly what I was looking for. Thank you!

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