String methods and list methods have differing behavior with and without variable assignment in Python

Question

Hopefully not a silly question. If I write

str1 = 'exterminate!'           

str1.upper()

print(str1)

str1 = str1.upper()

print(str1)

my_list = [1, 2, 3]

my_list.append(4)

print(my_list)

my_list = my_list.append(5)

print(my_list)

The output is:

exterminate!
EXTERMINATE!
[1, 2, 3, 4]
None

In other words,

str1.upper()

and

my_list.append(4)

do very different things. By that, I mean .append actually changes the list object, but .upper() does not.

However, in order to change the string object to all uppercase, we have to use

str1 = str1.upper()

but doing

my_list = my_list.append(5)

we now have my_list as a noneType.

Can someone explain this behavior. I have a feeling it has to do with the fact that they are different object types.

Have a look at the canonical explanation

Thierry Lathuille
– Thierry Lathuille

2017-06-26 12:22:08 +00:00
Commented Jun 26, 2017 at 12:22 — Thierry Lathuille
– Thierry Lathuille, Commented Jun 26, 2017 at 12:22

csl · Accepted Answer · 2017-06-26 11:40:33Z

3

Strings are immutable in Python.

Therefore, they don't offer in-place modification. The form foo = foo.upper() is therefore the usual way to do it for Python.

Note that a variable is not in itself immutable, meaning that when you do foo = foo.upper(), Python will return a new uppercase string based on the string pointed to by foo, then update the variable foo to point to that new string. (The old string may then be removed from memory, if its reference count goes to zero.)

Some other data structures in Python are also immutable, for example tuples like t = (1,2,3). To modify those, you'd need to construct new tuples.

answered Jun 26, 2017 at 11:40

csl

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Comments

Sven Marnach · Accepted Answer · 2017-06-26 11:40:59Z

2

Strings in Python are immutable. They can't be changed. This is why all the string methods return a new string instead of modifying the string in place. Lists on the other hand are mutable, and generally list methods mutate the list in place, and return None.

This is a general rule (with some exceptions): Mutating methods return None, while non-mutating methods return a new value.

answered Jun 26, 2017 at 11:40

Sven Marnach

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gonczor · Accepted Answer · 2017-06-26 11:41:01Z

0

upper() method returns a copy of a string. See docs for reference.

answered Jun 26, 2017 at 11:41

gonczor

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Mehrad.Mz · Accepted Answer · 2017-06-26 12:03:47Z

0

the .upper() method does not change anything in your String but it return a new string that is with the upper words and you need to save that output in a variable. but in .append() method add your input in the list and you don't need to use my_list = my_list.append(5)

answered Jun 26, 2017 at 12:03

Mehrad.Mz

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