Python's interpretation of tabs and spaces to indent

Question

I decided, that I learn a bit of Python. The first introduction says that it uses indentation to group statements. While the best habit is clearly to use just one of these what happens if I interchange them? How many spaces will be considered equal to one tab? Or will it fail to work at all if tabs and spaces are mixed?

Answer 1

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).

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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.

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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'.)

Answer 2

Follow PEP 8 for Python style. PEP 8 says: Indentation

Use 4 spaces per indentation level.

For really old code that you don't want to mess up, you can continue to use 8-space tabs.

Tabs or Spaces?

Never mix tabs and spaces.

The most popular way of indenting Python is with spaces only. The second-most popular way is with tabs only. Code indented with a mixture of tabs and spaces should be converted to using spaces exclusively. When invoking the Python command line interpreter with the -t option, it issues warnings about code that illegally mixes tabs and spaces. When using -tt these warnings become errors. These options are highly recommended!