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?](https://stackoverflow.com/questions/35488769/what-is-an-object-reference-in-python) — Mateen Ulhaq, Jun 27 '18 at 02:48
`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, Jun 27 '18 at 02:51
`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, Jun 27 '18 at 02:56

score 5 · Accepted Answer · answered Jun 27 '18 at 02:56

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.

Python clear list clearing wrong list?

1 Answers1