Found interesting thing in Python (2.7) that never mentioned before.
This:
a = []
a += "a"
does work and result is:
>>> a
>>> ["a"]
But
a = []
a = a + "a"
gives
>>> TypeError: can only concatenate list (not "str") to list
Can someone explain why? Thanks for your answers.
Python distinguishes between the + and += operators and provides separate hooks for these; __add__ and __iadd__. The list() type simply provides a different implementation for the latter.
It is more efficient for lists to implement these separately; __add__ has to return a completely new list, while __iadd__ can just extend self then return self.
In the C code, __iadd__ is implemented by list_inplace_concat(), which simply calls listextend(), or, in python code, [].extend(). The latter takes any sequence, by design.
The __add__ method on the other hand, represented in C by list_concat, only takes a list as input, probably for efficiency's sake; it can loop directly over the internal C array and copy items over to the new list.
In conclusion, the reason __iadd__ accepts any sequence is because when PEP 203 (the Augmented Add proposal) was implemented, for lists it was simplest just to reuse the .extend() method.
If a is a list, a + x only works if x is also list, whereas a += x works for any iterable x.
The following might help understand it:
In [4]: a = []
In [5]: a += "abc"
In [6]: a
Out[6]: ['a', 'b', 'c']
The key is that "a" and "abc" are iterable, which is what enables their use on the right-hand side of +=.
This doesn't work for + because the latter requires both operands to be of the same type (see the manual).
To write the same thing using +, you have to expand the iterable:
In [7]: a = []
In [8]: a = a + list("abc")
In [9]: a
Out[9]: ['a', 'b', 'c']
In other words, += is more general than + when applied to lists.
