Python 列表切片和 += 运算符

Question

Given the below examples:

array = [1,2,3,4,0]

In: array[0] += 2 
Out: 3

In: array[1:3] += 2  
Out: TypeError: 'int' object is not iterable

In: array[1:3] += [100, 100] 
Out: [1, 2, 3, 100, 100, 4, 5]

Can someone explain me why the two last examples wont return something like [1,102,103,4,0] AND if it is possible doing this with a simple slice, not using a for loop...

Answer 1

When using the slice operator it refers to sub-part of the list, so operating on it requires a list too (we can use the "add" operator on list with list , and not with an int , unlike in some other languages).

Therefore the following:

array[1:3] += 2

Throws:

TypeError: 'int' object is not iterable

Because 2 is not a list (actually an iterable, which is more general than list ).

But:

array[1:3] += [100, 100]

Works and adds (actually appends) the two elements in the middle (index 3 ) of array according to indexes:

[3, 2, 3, 100, 100, 4, 0]

Without using a for loop, as requested

If you want to add to the values in the slice:

array = [1,2,3,4,0]

array.__setitem__(slice(1, 3), [x+2 for x in array[1:3]])

# [1, 102, 103, 4, 0]
print(array)

Which can be written also as:

array = [1,2,3,4,0]

def apply_on_slice(lst, start, end, callable):
    array.__setitem__(slice(start, end), [callable(x) for x in array[start:end]])

apply_on_slice(array, 1, 3, lambda x : x + 100)

# [1, 102, 103, 4, 0]
print(array)

Using a for loop

Here are some other options to do so, elegantly:

array[1:3] = (x+2 for x in array[1:3])

Or of course, using a regular for loop, which is more efficient than using slicing twice:

for i in range(1, 3): 
    array[i] += 2

Answer 2

You are clearly expecting the operation to be applied element-wise, as in R and other languages (and within Python, in numpy arrays and the like). Eg, adding 2 to a list would add 2 to each of the list's elements. This is not how Python lists work: Each of the statements you ask about constructs one object on each side of the operator (a list or list slice, a list element, an integer), then applies the operation (just once) to these two objects. So if you "add" two lists you get concatenation, if you try to add a list and an int you get a TypeError, etc. The details you can read in @Aviv's answer.