str.expandtabs(n) is not equivalent to str.replace("\t", " " * n).

str.expandtabs(n) keeps track of the current cursor position on each line, and replaces each tab character it finds with the number of spaces from the current cursor position to the next tab stop. The tab stops are taken to be every n characters.

This is fundamental to the way tabs work, and is not specific to Python. See this answer to a related question for a good explanation of tab stops.

string.expandtabs(n) is equivalent to:

def expandtabs(string, n):
    result = ""
    pos = 0
    for char in string:
        if char == "\t":
            # instead of the tab character, append the
            # number of spaces to the next tab stop
            char = " " * (n - pos % n)
            pos = 0
        elif char == "\n":
            pos = 0
        else:
            pos += 1
        result += char
    return result

And an example of use:

>>> input = "123\t12345\t1234\t1\n12\t1234\t123\t1"
>>> print(expandtabs(input, 10))
123       12345     1234      1
12        1234      123       1

Note how each tab character ("\t") has been replaced with the number of spaces that causes it to line up with the next tab stop. In this case, there is a tab stop every 10 characters because I supplied n=10.