Python Variable assignment in a for loop
loops, python, variables
Solution
a=[1,2,3]
b=a
a.append(4)
print(b) #[1,2,3,4]
print(a) #[1,2,3,4]
Here you are modifying the list. The list content changes, but the list identity remains.
a=1
b=a
a+=1
This, however, is a reassignment. You assign a different object to `a`.
Note that if you did `a += [4]` in the 1st example, you would have seen the same result. This comes from the fact that `a += something` is the same as `a = a.__iadd__(something)`, with a fallback to `a = a.__add__(something)` if `__iadd__()` doesn't exist.
The difference is that `__iadd__()` tries to do its job "inplace", by modifying the object it works on and returning it. So `a` refers to the same as before. This only works with mutable objects such as lists.
On immutable objects such as ints `__add__()` is called. It returns a different object, which leads to `a` pointing to another object than before. There is no other choice, as ints are immutable.
a,b,c=1,2,3
li=[a,b,c]
for x in li:
x+=1
print(li) #[1,2,3]
Here `x += 1` means the same as `x = x + 1`. It changes where `x` refers to, but not the list contents.
Maybe my question should be how to loop over a list of integers and change them without >map() as i need a if statement in there.
for i, x in enumerate(li):
li[i] = x + 1
assigns to every list position the old value + 1.
Problem
I understand that in Python regular c++ style variable assignment is replaced by references to stuff ie ``` a=[1,2,3] b=a a.append(4) print(b) #gives [1,2,3,4] print(a) #gives [1,2,3,4] ``` but I'm still confused why an analogous situation with basic types eg. integers works differently? ``` a=1 b=a a+=1 print(b) # gives 1 print(a) # gives 2 ``` But wait, it gets even more confusing when we consider loops! ``` li=[1,2,3] for x in li: x+=1 print(li) #gives [1,2,3] ``` Which is what I expected, but what happens if we do: ``` a,b,c=1,2,3 li=[a,b,c] for x in li: x+=1 print(li) #gives [1,2,3] ``` Maybe my question should be how to loop over a list of integers and change them without map() as i need a if statement in there. The only thing I can come up short of using ``` for x in range(len(li)): Do stuff to li[x] ``` is packaging the integers in one element list. But there must be a better way.