I read this post: What's the difference between [a] + [b] and [a].extend([b])?
It says that in terms of lists "+" between lists creates a new list and extend changes the list which is operated on.
[1,2,3] + [4]
vs
[1,2,3].extend([4])
Is there any way to get this information without diving into the documentation, but only using the interpreter using help() or dir() for example?
Limiting yourself to the interpreter, one way to do it is through the instance method's docstring. You can access it through help(list.extend) or through the __doc__ property. For instance:
>>> list.extend.__doc__
'Extend list by appending elements from the iterable.'
As for the + operator merging two lists, that's only possible thanks to operator overloading by the list class. Behind the scenes, it's implemented through a special method called __add__. Using the same approach as above:
>>> list.__add__.__doc__
'Return self+value.'
That being said, the online docs are certainly a less convoluted way to access this information.
This might not be what you're after, but you can easily check for or verify this behavior from the interpreter.
>>> l1 = [1,2,3]
>>> l2 = [4]
>>> l1 + l2, l1, l2
([1,2,3,4], [1,2,3], [4])
>>> l1.extend(l2), l1, l2
(None, [1,2,3,4], [4])
A. If a function (method) modifies data and does not return the result, then is must have mutated objects passed to it as function inputs. This is called in-place modification.
Such functions often deliberately return nothing (None). This convention is nicely described at list.sort:
This method modifies the sequence in place ..... To remind users that it operates by side effect, it does not return the sorted sequence
Note that there exists a technique called "Fluent interface" based on method chaining requiring an opposite approach.
B. you can always check with id() if a new object was created
>>> l1 = [1,2,3,4]
>>> l2 = ['A','B','C']
>>> id(l1), id(l2), id(l1+l2)
(139855120180552, 139855123390792, 139855123880392)
