Question

I’m writing a Python parser to learn Flex and Bison, and I’m trying to find out why only the first of these programs is valid Python.

a.py:

\
# This is valid Python

produces no error.

b.py:

    \
# This is not valid Python

produces this error:

  File "b.py", line 1
    \
    ^
IndentationError: unexpected indent

and c.py:

if True:
    pass
    \
    # This is not valid Python

produces this error:

  File "c.py", line 4
    # This is not valid Python
                             ^
SyntaxError: invalid syntax

I’m using Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) [GCC 4.4.3] on linux2 (Ubuntu 10.04); However, testing on ideone.com suggests the behavior is the same on Python 3.

---

_{See also: How can I do a line break (line continuation) in Python (split up a long line of source code)?}

Answer 1

It’s an implementation detail.

Here’s how a few different implementations respond to your code:

                 a.py  b.py  c.py    
                 ----  ----  ----
CPython 2.6.5     ok    bad   bad
CPython 3.?       ok    bad   bad
Jython 2.2.1      ok    ok    bad
Jython 2.5.2      bad   bad   bad
IronPython 2.7.1  ok    bad   ok

My reading of the Exlplicit Line Joining section of the Python Language Reference is that all three examples could be treated as valid:

Two or more physical lines may be joined into logical lines using
backslash characters (\), as follows: when a physical line ends in a
backslash that is not part of a string literal or comment, it is
joined with the following forming a single logical line, deleting the
backslash and the following end-of-line character.

If CPython were changed to accept all three examples as valid I doubt it would be noticed by its users, change the character of the language, or break any code.