Variable variables in python for specific case

Question

Is there a way to create variable variables in Python for the following specific case: say I have a function that outputs lots of objects (a fixed, but large, number, say 20), like

a1, a2, a3, a4, ..., a20 = f(20),

where a1, a2, a3, ..., a20 are objects.

There have been many questions asked on this topic in SE, where people generally say to use lists or dictionaries. What I don't understand is how to use lists/dictionaries in this specific case.

If I had:

a1 = g(1)
a2 = g(2)
...

I would understand how to do this, but I'm not sure in the above case how to "capture" all the objects into a list without providing explicitly all the variable names.

Python function can return one value. So it's a little confusing when you say, "function that outputs lots of objects". Do you mean it outputs a list of objects? It would be really helpful if you could show a minimal example of your problem. — Mark
– Mark, Commented Nov 2, 2020 at 1:43
if your function returned lots of objects... they are already in a container - likely a tuple. You could do foo = f(20) and then, say, a2 would be foo[1]. — tdelaney
– tdelaney, Commented Nov 2, 2020 at 1:46
Ah, thanks! I was thinking about matplotlib's plt.subplots, with fig, (ax1, ax2, ax3, ...) = plt.subplot(). But I just looked at the documentation, and I can use fig, ax = plt.subplot() to get the whole thing instead. How do I accept the comment as an answer? — Physics Enthusiast
– Physics Enthusiast, Commented Nov 2, 2020 at 1:50

tdelaney · Accepted Answer · 2020-11-02 01:52:13Z

1

A python function always returns a single object. That object could be None, a single value, or a container holding values. When returning a container, python lets you unpack that container into multiple variables or keep it a single variable.

You can unpack

>>> def f(val):
...     return list(range(val))
... 
>>> a1, a2, a3 = f(3)

Or just keep the container

>>> foo = f(3)
>>> type(foo)
<class 'list'>
>>> foo
[0, 1, 2]
>>> a2 == foo[1]
True

answered Nov 2, 2020 at 1:52

tdelaney

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Carson · Accepted Answer · 2020-11-02 01:52:06Z

I'm not sure if you're asking how to access your 20 values once they're returned, or how to return 20 different values.

I will say, more as a point of advice, that most times you're trying to return a data structure with 20 parts, it might be easier to make that a class, and return a class.

But, that doesn't answer your question. For this example, assuming that you really just want a list of 20 things passed through, I suggest tuples. See the example below for a function that creates a tuple, and that same tuple being unpacked earlier.

# My function get_information returns the latitude,
# longitude, time, and number of likes a given post has

def get_post(post_id):
    # Get post information here
    long = 12.4213
    lat = 12351.34
    time = '12:00'
    num_likes = -3
    # Now to return all these values
    return ( long, lat, time, num_likes )

# Outside of the function, we can access the values in this way
my_long, my_lat, my_time, my_num_likes = get_post(10)
# Python unpacks the tuple into these variables in the order they are put into the tuple.

NotAName · Accepted Answer · 2020-11-02 01:55:58Z

1

Say, you have your function that returns a tuple of 20 objects, here you don't need to create many variables to hold these objects, but instead you can use a dictionary for example:

# Say it's your function
def f(x):
    return a0, a1, a2, ..., a19

Then you can easily place all return values in a dictionary like this:

foo = f(x)
dic = {f'a{i}': foo[i] for i in range(len(foo))}

And here you will get a dictionary like:

dic = {'a0': first_return_value,
       'a1': second_return_value,
       etc,}

answered Nov 2, 2020 at 1:55

NotAName

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