repr method and repr() on a list return different objects

Question

Say that we have a list my_num=[1,2,3,4,5]. Applying repr() on that list returns a representation of this object as a string, say a=repr(my_num). I thought that calling my_num.__repr__() would yield the same object.

However:

>>> my_num=[1,2,3,4,5]
>>> a=repr(my_num)
>>> a is my_num.__repr__()
False

Why is this happening?

What about `a = my_num.__repr__()`? `my_num.__repr__` is a function, actually is the function that is called when you do `repr(my_num)` — Dani Mesejo, Mar 20 '20 at 17:51
I tried that too, didn't work. In fact I changed my question and included the parenthesis. — user430191, Mar 20 '20 at 17:53
yeah strictly speaking you're right. I'm talking about the underlying structure of these two objects. — user430191, Mar 20 '20 at 17:57
Both are strings that have the same value, but they are not the same string — Dani Mesejo, Mar 20 '20 at 17:58

score 2 · Answer 1 · answered Mar 20 '20 at 17:59

Very simply, each call returns a string image of my_num. Python will fold small integers, but not a string of any complexity. Each call returns a separate string object, so they have different id values.

is compares id's; == compares values.

>>> my_num=[1,2,3,4,5]
>>> repr(my_num)
'[1, 2, 3, 4, 5]'
>>> a = repr(my_num)
>>> id(a)
139786829689392
>>> a
'[1, 2, 3, 4, 5]'
>>> b = my_num.__repr__()
>>> b
'[1, 2, 3, 4, 5]'
>>> id(b)
139786829689264
>>> a is b
False
>>> a == b
True

score 1 · Answer 2 · answered Mar 20 '20 at 17:58

1

>>> my_num = [1, 2, 3, 4, 5]
>>> id(repr(my_num))
140357224404912
>>> id(my_num.__repr__())
140357224404976
>>> repr(my_num) == my_num.__repr__()
True

As you can see id is different (i.e. these are different objects), but they are equal.

answered Mar 20 '20 at 17:58

buran

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Alonme · Answer 3 · 2020-03-20T18:08:37.827

There is a difference between equality and identity. using repr(obj) and obj.__repr__() is the same, however the repr function creates a new string object, and hence the two objects that are created are not identical.

You can even see that:

>>> my_num=[1,2,3,4,5]
>>> my_num.__repr__() is my_num.__repr__()
False

Check these for more info:

Python: Why does ("hello" is "hello") evaluate as True?

Why does comparing strings using either '==' or 'is' sometimes produce a different result?

__repr__ method and repr() on a list return different objects

3 Answers3

repr method and repr() on a list return different objects