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Date: Wed, 18 Mar 2015 13:15:28 -0500
From: Tim Chase <python.list@tim.thechases.com>
To: Aditya Raj Bhatt <adityarajbhatt@gmail.com>
Cc: python-list@python.org
Subject: Re: A simple single line, triple-quoted comment is giving syntax error. Why?
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On 2015-03-18 10:46, Aditya Raj Bhatt wrote:
> a = 5 '''a comment'''
> 
> results in a syntax error

That's to be expected, and happens with any string, not just
triple-quoted:

  >>> a = 5 "hello"

> there are no 'true' multiline comments in python and that all those
> 'block' comments are actually triple-quoted strings.

Yes, that's an abuse of strings.  It works nicely for doc-strings,
but I avoid it for everything else.

> So can someone tell me why a triple-quoted string gives a syntax
> error if only in one line?

When a string (triple-quoted or otherwise) begins at the beginning of
a line, its return value is ignored at compile-time:

>>> def a():
...     print 1
...     "test"
...     print 2
... 
>>> import dis
>>> dis.dis(a)
  2           0 LOAD_CONST               1 (1)
              3 PRINT_ITEM          
              4 PRINT_NEWLINE       

  4           5 LOAD_CONST               2 (2)
              8 PRINT_ITEM          
              9 PRINT_NEWLINE       
             10 LOAD_CONST               0 (None)
             13 RETURN_VALUE        

Note that nothing appears in the byte-code for line #3.  They Python
interpreter/compiler is smart enough to know that the string can be
discarded.

> Actually, there are other confusions I have too, regarding using
> backslashes inside triple-quoted strings to form multi-line
> comments, and a general uncertainty about triple-quoted strings.
>
> Can someone also provide a sort of a 'guide' to triple-quoted
> comments in general?

Triple-quoted strings aren't particularly magical other than that
they allow you to have multi-line strings and it allows you to have
both single and double quotes inside the string with less mess than
if you have them mixed in a regular string (where you'd need to
escape at least one of them).

  s1 = "Single ' and double \" quotes"
  s2 = 'Single \' and double " quotes'
  s3 = """Single ' and double " quotes"""

If you want to avoid some of the backslash issues, you can make it a
raw triple-quoted string:

  s = r"""this is in
  c:\temp\new>
  for you to test
  """

Note the leading "r" before the quotes.  This is what tells Python
not to translate the "\t" and "\n" in the following content.

-tkc