A tab is an actual character “\t”. It’s usually represented as a space of length 4, but is still a single distinct character. 4 spaces is by far the preferred standard, but no one actually types 4 spaces each time, all but the most extremely basic text editors have a setting to convert tabs to spaces. This is what I and most people would suggest.

Spaces are not treated as equivalent to tab. A line indented with a tab is at a different indentation from a line indented with 1, 2, 4 or 8 spaces.

Proof by counter-example (erroneous, or, at best, limited - tab != 4 spaces):

x = 1
if x == 1:
^Iprint "fff\n"
    print "yyy\n"

The '^I' shows a TAB. When run through Python 2.5, I get the error:

  File "xx.py", line 4
    print "yyy\n"
                ^
IndentationError: unindent does not match any outer indentation level

Thus showing that in Python 2.5, tabs are not equal to spaces (and in particular not equal to 4 spaces).

Oops - embarrassing; my proof by counter-example shows that tabs are not equivalent to 4 spaces. As Alex Martelli points out in a comment, in Python 2, tabs are equivalent to 8 spaces, and adapting the example with a tab and 8 spaces shows that this is indeed the case.

x = 1
if x != 1:
^Iprint "x is not 1\n"
        print "y is unset\n"

In Python 2, this code works, printing nothing.

In Python 3, the rules are slightly different (as noted by Antti Haapala). Compare:

• Python 2 on Indentation
• Python 3 on Indentation

Python 2 says:

First, tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight (this is intended to be the same rule as used by Unix). The total number of spaces preceding the first non-blank character then determines the line’s indentation. Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation.

Python 3 says:

Tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight (this is intended to be the same rule as used by Unix). The total number of spaces preceding the first non-blank character then determines the line’s indentation. Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation.

(Apart from the opening word "First," these are identical.)

Python 3 adds an extra paragraph:

Indentation is rejected as inconsistent if a source file mixes tabs and spaces in a way that makes the meaning dependent on the worth of a tab in spaces; a TabError is raised in that case.

This means that the TAB vs 8-space example that worked in Python 2 would generate a TabError in Python 3. It is best — necessary in Python 3 — to ensure that the sequence of characters making up the indentation on each line in a block is identical. PEP8 says 'use 4 spaces per indentation level'. (Google's coding standards say 'use 2 spaces'.)

Everyone else uses 4 spaces. That is the only reason to use 4 spaces that I've come across and accepted. In my heart, I still want to use tabs (1 indent character per indent, makes sense, no? Separate indent from other whitespace. I don't care that tabs can be displayed as different widths, that makes no syntactic difference. The worst that can happen is that some of the comments don't line up. The horror!) but I've accepted that since the python community as a whole uses 4 spaces, I use 4 spaces. This way, I can assemble code from snippets others have written, and it all works.

Because the default tab size in Linux console is 8 spaces, and therefore most CLI text editors in Linux also default to 8 spaces. Most are also configurable, but it's been the default for ages.

Some older (C) code uses mixed indention to fake a 4 space tab - 1 indent == 4 spaces, 2 indents == 1 tab, 3 indents == 1 tab + 4 spaces, etc..... it was awful. Not sure if it was done intentionally in order to make the code easier to read, or if some editor did this automatically to simulate 4 space tabs. All I know is that I was using pico and it was a PITA working with those, especially when you needed to indent/dedent a whole block. :)