为什么这个参数列表在 Python 中没有变化？

Question

I created a function f which uses a 2-dimension list as parameter, but after this function the list does not change at all. As the code below:

def f(t: [[int]]):
    for eachrow in t:
        eachrow = eachrow[1:]
        eachrow.append(0)

A = [[2, 10, 0], [3, 1, 2], [3, 2, 1]]

f(A)

print(A)  # -> [[2, 10, 0], [3, 1, 2], [3, 2, 1]]

Answer 1

Assigning to eachrow in eachrow = eachrow[1:] overwrites it. So to remove the first element, you could use del instead, or row.pop or slice assignment .

def f(t):
    for row in t:
        del row[0]  # OR row.pop(0) OR row[:] = row[1:]
        row.append(0)

A = [[2, 10, 0], [3, 1, 2], [3, 2, 1]]
f(A)
print(A)  # -> [[10, 0, 0], [1, 2, 0], [2, 1, 0]]

Answer 2

If you print out the results of your changes to eachrow , you'll see that you ARE updating the eachrow variable, but that doesn't affet the original t variable.

def f(t):
    for eachrow in t:
        eachrow = eachrow[1:]
        eachrow.append(0)
        print(eachrow)

>>> f(A)
[10, 0, 0]
[1, 2, 0]
[2, 1, 0]

If you want to affect the array itself, you should modify the array like so:

def f(t):
    for row_number in range(len(t)):
        t[row_number] = t[row_number][1:]
        t[row_number].append(0)