Python clear list clearing wrong list?

Question

So I have a very rudimentary understanding of python (10-week summer course split up between python and Matlab). I am trying to create a 2d list like so:

data.append (samples)
data.append (matches)
data_list.append (data)
data.clear()

This is running in a for loop and writing each time it iterates. However, when I run it and I print(data_list) I get an empty list.

I have successfully ran it like so:

data.append (samples)
data.append (matches)
data_list.append (data)
data = []

But I do not understand the difference between my two methods and why only the second one works...

Possible duplicate of What is an object reference in Python? — Mateen Ulhaq
– Mateen Ulhaq, Commented Jun 27, 2018 at 2:48
data_list.append(data.copy()) will solve your issue. The problem is you're putting a reference to data inside data_list, not the stuff inside data. So when you clear data, the reference inside data_list is now pointing to an empty list. — alkasm
– alkasm, Commented Jun 27, 2018 at 2:51
samples and matches are integer values. Its a program that compares your birthday to a random sample group. So I understand how the data_list.append(data.copy()) solves this issue. It is strange that python handles that differently, because by clearing data by data=() is preserves the values in data_list however clearing by .clear it doesn't preserve the data. — stueck9356
– stueck9356, Commented Jun 27, 2018 at 2:56

Mateen Ulhaq · Accepted Answer · 2018-06-27 02:56:55Z

In Python, it is critical to understand that everything is an object. Of these, you have two types:

immutable (int, float, char, str, tuple)
mutable (list, dict, other objects, ...)

Immutable objects cannot be changed. That is, you cannot redefine a immutable:

1 = 0

That is ridiculous! You may, however, bind immutables to names. We call such names "variables".

x = 1
y = x
x = 0

# x = 0
# y = 1

Mutable objects, however, may change their internal contents:

x = [0, 2]
y = x
x[0] = 1

# x = [1, 2]
# y = [1, 2]

Notice that y is merely a name to the list. When you mutate the list x with x[0] = 1, you are mutating the same list that y is bound to.

Why am I telling you all this? Because:

data.clear()

mutates the original list that data is bound to.

In contrast,

data = ()

simply rebinds data to some other object.

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