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A simple single line, triple-quoted comment is giving syntax error. Why?

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  A simple single line, triple-quoted comment is giving syntax error. Why? Aditya Raj Bhatt <adityarajbhatt@gmail.com> - 2015-03-18 10:46 -0700
    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Laurent Pointal <laurent.pointal@laposte.net> - 2015-03-18 19:04 +0100
      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Laurent Pointal <laurent.pointal@laposte.net> - 2015-03-18 19:06 +0100
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-18 20:53 +0100
          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-18 20:56 +0100
          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Terry Reedy <tjreedy@udel.edu> - 2015-03-18 17:47 -0400
          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-19 09:58 +1100
            Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-03-19 10:23 +1100
      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Aditya Raj Bhatt <adityarajbhatt@gmail.com> - 2015-03-18 11:23 -0700
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Igor Korot <ikorot01@gmail.com> - 2015-03-18 14:45 -0400
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Laurent Pointal <laurent.pointal@laposte.net> - 2015-03-18 20:08 +0100
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Tim Roberts <timr@probo.com> - 2015-03-28 16:34 -0700
    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-18 21:21 +0100
    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Tim Chase <python.list@tim.thechases.com> - 2015-03-18 13:15 -0500
    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ben Finney <ben+python@benfinney.id.au> - 2015-03-19 11:06 +1100
    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Denis McMahon <denismfmcmahon@gmail.com> - 2015-03-19 00:53 +0000
      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-22 04:14 +0100
        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-03-22 14:36 +1100
          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-22 04:49 +0100
            Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-03-22 15:11 +1100
              Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 05:35 +0100
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-25 23:09 -0600
                  Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 17:45 +0100
                    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-26 12:29 -0600
                      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 19:54 +0100
                        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-26 17:27 -0600
                          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-28 19:20 +0100
                            Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-29 01:56 -0600
                              Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-29 12:41 +0200
                                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-31 01:23 -0600
                                  Re: A simple single line, triple-quoted comment is giving syntax error. Why? sohcahtoa82@gmail.com - 2015-03-31 16:10 -0700
                                    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-04-01 10:36 +1100
                                    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-31 17:43 -0600
                                  Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-04-02 23:31 +0200
                                    Re: A simple single line, triple-quoted comment is giving syntax error. Why? sohcahtoa82@gmail.com - 2015-04-02 15:26 -0700
                                    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-02 18:21 -0600
                                    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-04-03 16:40 +1100
                                      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Chris Angelico <rosuav@gmail.com> - 2015-04-03 16:57 +1100
                                        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Marko Rauhamaa <marko@pacujo.net> - 2015-04-03 10:13 +0300
                                          r"\"" ??? (was A simple single line, triple-quoted comment) Rustom Mody <rustompmody@gmail.com> - 2015-04-03 05:52 -0700
                                            Re: r"\"" ??? (was A simple single line, triple-quoted comment) Chris Angelico <rosuav@gmail.com> - 2015-04-04 01:40 +1100
                                              Re: r"\"" ??? (was A simple single line, triple-quoted comment) Rustom Mody <rustompmody@gmail.com> - 2015-04-03 08:14 -0700
                                          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-03 09:25 -0600
                                      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-04-03 00:40 -0600
                      Re: A simple single line, triple-quoted comment is giving syntax   error. Why? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-03-27 11:43 +1300
                        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-28 19:06 +0100
                    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-26 12:32 -0600
                      Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 19:56 +0100
                        Re: A simple single line, triple-quoted comment is giving syntax error. Why? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-27 10:15 +1100
                          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Tim Chase <python.list@tim.thechases.com> - 2015-03-26 18:47 -0500
                          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Ian Kelly <ian.g.kelly@gmail.com> - 2015-03-26 19:38 -0600
                          Re: A simple single line, triple-quoted comment is giving syntax error. Why? Marko Rauhamaa <marko@pacujo.net> - 2015-03-27 07:34 +0200
                Re: A simple single line, triple-quoted comment is giving syntax error. Why? Dave Angel <davea@davea.name> - 2015-03-26 01:23 -0400
                  Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 18:25 +0100
                    Re: A simple single line, triple-quoted comment is giving syntax error. Why? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2015-03-26 18:30 +0100
            Re: A simple single line, triple-quoted comment is giving syntax error. Why? Marko Rauhamaa <marko@pacujo.net> - 2015-03-22 12:00 +0200

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#87687 — A simple single line, triple-quoted comment is giving syntax error. Why?

I always do single line comments with # but just for the sake of it I
tried it with ''' ''' and it gives me a syntax error.

In both the interpreter, and the source code text file, doing -

a = 5 '''a comment'''

results in a syntax error, with the very last quote at the end of the
line highlighted in red. Of course, if I do -

a = 5 #'''a comment'''

it works. I searched for the problem, and arrived at
http://stackoverflow.com/questions/397148/why-doesnt-python-have-multiline-comments
which says that there are no 'true' multiline comments in python and
that all those 'block' comments are actually triple-quoted strings. Then
I looked in the documentation and found
https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals
but it is a little bit too complex for my understanding (I'm just
starting python). 

So can someone tell me why a triple-quoted string gives a syntax error
if only in one line? 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?
Something like how I can just sum up index slices by saying in [a:b],
the 'counting' for a always starts with 0, a is included, everything up
to b but not b is included (assuming this is in fact the correct
explanation ;-))

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#87688

Aditya Raj Bhatt wrote:

> I always do single line comments with # but just for the sake of it I
> tried it with ''' ''' and it gives me a syntax error.
> 
> In both the interpreter, and the source code text file, doing -
> 
> a = 5 '''a comment'''
> 
> results in a syntax error, with the very last quote at the end of the
> line highlighted in red. Of course, if I do -
> 
> a = 5 #'''a comment'''
<zip>
> Can someone also provide a sort of a 'guide' to triple-quoted comments
> in general?

A triple ' or " string is a Python string, allowing line-return in string.

If it is in an expression (like a = 5 '''a comment'''), then it must be a 
valid expression (and here it is not).

You can have an expression alone, with no stored result (ie. no assignment 
to a name), and this is how are used triple quoted strings as comments.

a = 5
"""a comment"""

Take care of indent:

def f(x):
    a = 5
    """an correctly indented expression to be
    inside the function"""
    return a * x

A+
Laurent.

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#87690

Laurent Pointal wrote:

(oups)
> Take care of indent:
> 
> def f(x):
>     a = 5
>     """an correctly indented expression to be
>     inside the function"""
>     return a * x

Here only the first indent of """ at beginning of the string to be aligned 
to function bloc is important, remaining content of the string can be 
indented or not.

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#87696

Laurent Pointal wrote:

> Laurent Pointal wrote:
>> Take care of indent:
>> 
>> def f(x):
>>     a = 5
>>     """an correctly indented expression to be
>>     inside the function"""
>>     return a * x
> 
> Here only the first indent of """ at beginning of the string to be aligned
> to function bloc is important, remaining content of the string can be
> indented or not.

I must strongly object and recommend against getting accustomed to the 
suggested use of multi-line string literals.  First of all, it is too easy 
to produce invalid code this way.  Also, standalone multi-line string 
literals in Python code serve a specific purpose when at the beginning of a 
function, method or class clock: they constitute *docstrings”, setting the 
respective object’s “__doc__” attribute:

>>> def f(x):
...    """Returns a multiple of 5"""
...    a = 5
...    return a * x
... 
>>> print(f.__doc__)
Returns a multiple of 5

This means that different to real comments, the content of string literals 
is *parsed*.  You do not want to waste the time of the compiler compiling 
code that serves no purpose; particularly not in a language like Python 
where code is JIT-compiled by default.

Not any less important: Using standalone multi-line string literals in any 
other way is syntactically correct indeed, but makes the code harder to 
read.  Keep in mind that code may be written only once, but read many times 
by other people during its lifetime.  It is important to write code so that 
it is easy to read, so that it can be easily reused and adapted by oneself 
and others (a month, a year later).

If you want to comment your code, then by all means do so using docstrings 
and *real* comments.  IDEs like PyDev support you there.  That way also the 
syntax highlighting will be correct.  And with real comments, the Python 
compiler does not care about the indentation.  As to where you should put 
comments, I recommend in general, in all programming languages, if possible 
to put the comments on their own line, *before* the statement that they are 
describing, and using the same indentation, preferably preceded by an empty 
line (so that the comment and the code it documents stand out).

<https://docs.python.org/3/tutorial/controlflow.html#documentation-strings>
<https://docs.python.org/3/reference/lexical_analysis.html#line-structure>

-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

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#87697

Thomas 'PointedEars' Lahn wrote:

> […] standalone multi-line string literals in Python code serve a specific
> purpose when at the beginning of a function, method or class clock: they 
                                                               ^ block¹
> constitute *docstrings”, setting

_____
¹  Apparently I am doing to much relativity lately ;-)

-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

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#87702

On 3/18/2015 3:53 PM, Thomas 'PointedEars' Lahn wrote:

> I must strongly object and recommend against getting accustomed to the
> suggested use of multi-line string literals.

I agree.

> If you want to comment your code, then by all means do so using docstrings
> and *real* comments.  IDEs like PyDev support you there.

For instance, with Idle, before (with red comment coloring):

# this is
# a very very very ragged ragged ragged ragged ragged ragged ragged
# multiline comment
# that I will reformat
# with Idle's Format Paragraph (default key binding alt-Q, but this may 
be reset by the user as long as the user is careful to not resuse 
another key.

after alt-q anywhere in the comment block, with default 72 char lines

# this is a very very very ragged ragged ragged ragged ragged ragged
# ragged multiline comment that I will reformat with Idle's Format
# Paragraph (default key binding alt-Q, but this may be reset by the
# user as long as the user is careful to not resuse another key.

> That way also the syntax highlighting will be correct.


-- 
Terry Jan Reedy

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#87706

On Thu, 19 Mar 2015 06:53 am, Thomas 'PointedEars' Lahn wrote:

> I must strongly object and recommend against getting accustomed to the
> suggested use of multi-line string literals.

I agree in general with Thomas' comments about using arbitrary strings as
comments. If you want to write code according to *best practices*, then you
should not use strings as comments.

But let's also remember that sometimes you cannot be bothered with *best*
practice, and "good enough for now" practice is fine.

Need to comment out a large block of code in a hurry while using a basic
editor that doesn't offer a "Comment" and "Uncomment" command? Best
practice says I should use a better editor, and that's fine, but right this
instant I'm using the tools I have, not the tools I want, and the easiest
way to fix this is to surround the block of code with triple-quotes. I can
always come back and fix it later. Famous last words, I know, but who among
us can truthfully say they have *never* taken a quick and dirty short-cut
while programming?

I also find it convenient and unobjectionable to include a bare triple
quoted string at the very end of the file, as a sort of "note pad" area for
me to write notes to myself while the work is in early stages of progress.
I can put in small code snippets, URLs to pages I need to look at, general
comments, etc. without having to care about leading # signs. And because I
don't use strings for comments anywhere else, this note pad area stands out
and reminds me to remove it before releasing the code into production.


-- 
Steven

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#87709

On Thu, Mar 19, 2015 at 9:58 AM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> But let's also remember that sometimes you cannot be bothered with *best*
> practice, and "good enough for now" practice is fine.
>
> Need to comment out a large block of code in a hurry while using a basic
> editor that doesn't offer a "Comment" and "Uncomment" command? Best
> practice says I should use a better editor, and that's fine, but right this
> instant I'm using the tools I have, not the tools I want, and the easiest
> way to fix this is to surround the block of code with triple-quotes. I can
> always come back and fix it later. Famous last words, I know, but who among
> us can truthfully say they have *never* taken a quick and dirty short-cut
> while programming?

There are three broad types of comments:

1) Single-line or to-end-of-line comments, started with a # or a // or
something. Python supports these, they're easy to work with and
recognize. No problem.
2) Block comments, consisting of a number of entire lines of either
text or commented-out code.
3) Partial-line comments - /* comment */ in C-like languages.

You can abuse triple-quoted strings to do #2, but not #3. For
instance, in C, you can legally do something like this:

if (condition1 /* && condition2 */) do_stuff();

In terms of "quick and dirty short-cuts", that's one that I miss
having a mechanic for in Python. And yes, you can decry it as a bad
practice, but just as with Steven's example, it's always with the plan
to fix it later (usually by making the code more flexible in some
way), and yet does sometimes stay in the code for a rather long time,
for the same reason that several of my Alice posters are currently
lying face down on the Alice Shelf instead of being up on the wall -
there's nothing more permanent than a temporary solution.

(There's also technically a fourth use of slash-star comments -
beginning part way into one line and ending part way into another
line:

code code /* blah blah
blah blah
blah */ more code

But if you do this on a regular basis, I think we are justified in
throwing rotten fruit at you.)

ChrisA

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#87692

On Wednesday, March 18, 2015 at 1:04:39 PM UTC-5, Laurent Pointal wrote:
> > Can someone also provide a sort of a 'guide' to triple-quoted
comments
> > in general?
> 
> A triple ' or " string is a Python string, allowing line-return in
string.

What do you mean by line-return in string? Is it newline? Does it mean I
can write -

'''first part
second part'''

?
 
> If it is in an expression (like a = 5 '''a comment'''), then it must
be a 
> valid expression (and here it is not).

What is not a valid expression here?
 
><snip> 
> A+
> Laurent.

Thank you for the fast and detailed response,
Aditya

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#87693

 Hi,

On Wed, Mar 18, 2015 at 2:23 PM, Aditya Raj Bhatt
<adityarajbhatt@gmail.com> wrote:
> On Wednesday, March 18, 2015 at 1:04:39 PM UTC-5, Laurent Pointal wrote:
>> > Can someone also provide a sort of a 'guide' to triple-quoted
> comments
>> > in general?
>>
>> A triple ' or " string is a Python string, allowing line-return in
> string.
>
> What do you mean by line-return in string? Is it newline? Does it mean I
> can write -
>
> '''first part
> second part'''
>
> ?
>
>> If it is in an expression (like a = 5 '''a comment'''), then it must
> be a
>> valid expression (and here it is not).
>
> What is not a valid expression here?

a = 5 #Valid expression
b = c + d # Valid expression
a = 5 b = c + d # Invalid expression

a = 5 # Valid expression
''' testing comment ''' # Valid expression
a = 5 ''' testing comment ''' #Invalid expression

Thank you.
>
>><snip>
>> A+
>> Laurent.
>
> Thank you for the fast and detailed response,
> Aditya
> --
> https://mail.python.org/mailman/listinfo/python-list

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#87694

Aditya Raj Bhatt wrote:

> On Wednesday, March 18, 2015 at 1:04:39 PM UTC-5, Laurent Pointal wrote:
>> > Can someone also provide a sort of a 'guide' to triple-quoted
> comments
>> > in general?
>> 
>> A triple ' or " string is a Python string, allowing line-return in
> string.
> 
> What do you mean by line-return in string? Is it newline? Does it mean I
> can write -
> 
> '''first part
> second part'''
> 
> ?

Yes.

>> If it is in an expression (like a = 5 '''a comment'''), then it must
> be a
>> valid expression (and here it is not).
> 
> What is not a valid expression here?

What do you expect as result of this combination of an integer with a 
string?

>>> a = 5 '''a comment'''
  File "<stdin>", line 1
    a = 5 '''a comment'''
                        ^
SyntaxError: invalid syntax

You may use an *, and you buid a valid expression with a result

>>> a = 5 * '''a comment'''

But… this may not be the value you wanted for a variable.

>>> a
'a commenta commenta commenta commenta comment'

So, a string is really not a comment, even if triple quote strings can be 
used alone to store multi-line "comments".

A+
Laurent.

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#88238

Aditya Raj Bhatt <adityarajbhatt@gmail.com> wrote:

>On Wednesday, March 18, 2015 at 1:04:39 PM UTC-5, Laurent Pointal wrote:
>> > Can someone also provide a sort of a 'guide' to triple-quoted
>> > comments in general?
>> 
>> A triple ' or " string is a Python string, allowing line-return in
>> string.
>
>What do you mean by line-return in string? Is it newline? Does it mean I
>can write -
>
>'''first part
>second part'''
>
>?
> 
>> If it is in an expression (like a = 5 '''a comment'''), then it must
>> be a 
>> valid expression (and here it is not).
>
>What is not a valid expression here?

Were you ever able to extract the Real Answer to your original question
from the froth that resulted?  Your basic misunderstanding is that the
triple-quote thing is NOT a comment marker.  It is a string literal,
exactly like a single-quoted string, except that it allows embedded
newlines.  So, your statement
    a = 5 '''a comment'''
is invalid for exactly the same reason that the statement
    a = 5 "a comment"
is invalid.  Neither one of those statement have any comments.

There is a CONVENTION to embed a literal string as the first line in a
function, to allow for automatic documentation.  Whether the literal string
is single-quoted or triple-quoted is irrelevant.  That is, these two things
are equivalent:

    def func(a):
        "This is a function"
        return a*2

    def func(a):
        """This is a function"""
        return a*2
-- 
Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.

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#87698

Aditya Raj Bhatt wrote:

> I always do single line comments with # but just for the sake of it I
> tried it with ''' ''' and it gives me a syntax error.
> 
> In both the interpreter, and the source code text file, doing -
> 
> a = 5 '''a comment'''
> 
> results in a syntax error, with the very last quote at the end of the
> line highlighted in red. […]
> 
> So can someone tell me why a triple-quoted string gives a syntax error
> if only in one line?

As with all syntax errors in all programming languages, the code cannot be 
produced by the grammar (here: 
<https://docs.python.org/3/reference/grammar.html>, assuming Python 3.x).

> 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?

Do not use them as comments in this way.  See also my other follow-up.

[x] done

> Something like how I can just sum up index slices by saying in [a:b],
> the 'counting' for a always starts with 0, a is included, everything up
> to b but not b is included (assuming this is in fact the correct
> explanation ;-))

It is correct:

<https://docs.python.org/3/tutorial/introduction.html#strings>
<https://docs.python.org/3/tutorial/introduction.html#lists>
<https://docs.python.org/3/tutorial/datastructures.html#more-on-lists>
<https://docs.python.org/3/library/stdtypes.html#typesseq>

-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

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#87700

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

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#87711

Aditya Raj Bhatt <adityarajbhatt@gmail.com> writes:

> I always do single line comments with # but just for the sake of it I
> tried it with ''' ''' and it gives me a syntax error.

The only comment syntax in Python code is the line-end ‘# …’ syntax.

> In both the interpreter, and the source code text file, doing -
>
> a = 5 '''a comment'''

There is no comment on that line. You have an assignment statement
immediately followed by a string literal, which is invalid syntax.

> http://stackoverflow.com/questions/397148/why-doesnt-python-have-multiline-comments
> which says that there are no 'true' multiline comments in python and
> that all those 'block' comments are actually triple-quoted strings.

That's correct.

> So can someone tell me why a triple-quoted string gives a syntax error
> if only in one line?

Because the string literal – it doesn't matter how it's quoted – can
only appear where the syntax allows for a string literal. It isn't a
comment, so it can't be freely substituted for a comment.

> Can someone also provide a sort of a 'guide' to triple-quoted comments
> in general?

The guide is simple: There are no triple-quoted comments in Python.

If you triple-quote a string literal, it is still a string literal and
must follow all the syntax rules for string literals.

-- 
 \     “[The RIAA] have the patience to keep stomping. They're playing |
  `\         whack-a-mole with an infinite supply of tokens.” —kennon, |
_o__)                                             http://kuro5hin.org/ |
Ben Finney

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#87712

On Wed, 18 Mar 2015 10:46:20 -0700, Aditya Raj Bhatt wrote:

> I always do single line comments with # but just for the sake of it I
> tried it with ''' ''' and it gives me a syntax error.

> ...

> So can someone tell me why a triple-quoted string gives a syntax error
> if only in one line?

A triple quoted string is a multiline string literal. A string literal 
(of any sort) is a basic python expression.

An expression is often, but by no means exclusively, part of an 
assignment statement. However, an expression may also exist just as a 
simple expression.

There is nothing special about a triple quoted string that makes it a 
comment, other than it is sometimes used as such in it's guise as a basic 
expression.

However, you can't have multiple expressions on a line without some sort 
of operand or separator between them.

a = 5 '''text'''

is just as wrong as:

q = 4,5,6   [3,5,7,9]

or

k = 6-2  {56:91, 'fred': 'peter'}

or even

m = 62.3   56.7   101.2

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#87810

Denis McMahon wrote:

> However, you can't have multiple expressions on a line without some sort
> of operand or separator between them.

String concatenation is implicit in Python, but only with string *literals*:

| $ python
| Python 2.7.9 (default, Mar  1 2015, 12:57:24) 
| [GCC 4.9.2] on linux2
| Type "help", "copyright", "credits" or "license" for more information.
| >>> 'foo' 'bar'
| 'foobar'

| $ python3
| Python 3.4.2 (default, Dec 27 2014, 13:16:08) 
| [GCC 4.9.2] on linux
| Type "help", "copyright", "credits" or "license" for more information.
| >>> 'foo' 'bar'
| 'foobar'

They do not even have to be on the same line:

| $ python <<'EOT'
| print("foo"
|   "bar")
| EOT
| foobar

| $ python3 <<'EOT'
| print("foo"
|   "bar")
| EOT
| foobar

(I wished other programming languages had this feature.)

-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

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#87811

On Sun, Mar 22, 2015 at 2:14 PM, Thomas 'PointedEars' Lahn
<PointedEars@web.de> wrote:
> Denis McMahon wrote:
>
>> However, you can't have multiple expressions on a line without some sort
>> of operand or separator between them.
>
> String concatenation is implicit in Python, but only with string *literals*:

Which aren't expressions. Implicit concatenation is part of the
syntax, not part of the expression evaluator.

https://docs.python.org/2/reference/lexical_analysis.html#string-literal-concatenation
https://docs.python.org/3/reference/lexical_analysis.html#string-literal-concatenation

> (I wished other programming languages had this feature.)

But I absolutely agree. A handful do (Pike, with the same semantics as
Python; REXX, but with the proviso that any amount of whitespace
between the literals becomes a single space in the resulting string),
and it really is a very handy feature.

ChrisA

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#87812

Chris Angelico wrote:

> On Sun, Mar 22, 2015 at 2:14 PM, Thomas 'PointedEars' Lahn
> <PointedEars@web.de> wrote:
>> Denis McMahon wrote:
>>> However, you can't have multiple expressions on a line without some sort
>>> of operand or separator between them.
>> String concatenation is implicit in Python, but only with string
>> *literals*:
> 
> Which aren't expressions.

IBTD:

,-<https://docs.python.org/3/reference/grammar.html>
| 
| […]
| expr: xor_expr ('|' xor_expr)*
| xor_expr: and_expr ('^' and_expr)*
| and_expr: shift_expr ('&' shift_expr)*
| shift_expr: arith_expr (('<<'|'>>') arith_expr)*
| arith_expr: term (('+'|'-') term)*
| term: factor (('*'|'/'|'%'|'//') factor)*
| factor: ('+'|'-'|'~') factor | power
| power: atom trailer* ['**' factor]
| atom: ('(' [yield_expr|testlist_comp] ')' |
|        '[' [testlist_comp] ']' |
|        '{' [dictorsetmaker] '}' |
|        NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False')

> Implicit concatenation is part of the syntax, not part of the expression 
> evaluator.

Reads like nonsense to me.
 
-- 
PointedEars

Twitter: @PointedEars2
Please do not cc me. / Bitte keine Kopien per E-Mail.

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#87813

On Sun, Mar 22, 2015 at 2:49 PM, Thomas 'PointedEars' Lahn
<PointedEars@web.de> wrote:
>> Implicit concatenation is part of the syntax, not part of the expression
>> evaluator.
>
> Reads like nonsense to me.

What do you mean? String concatenation by abuttal is as much a
syntactic element as the distinction between regular, raw, and
triple-quoted string literals. By the time you get to AST (never mind
about byte code), that information is gone:

>>> print(ast.dump(ast.parse("""
... x = '''hello'''
... """)))
Module(body=[Assign(targets=[Name(id='x', ctx=Store())], value=Str(s='hello'))])
>>> print(ast.dump(ast.parse("""
... x = r'hello'
... """)))
Module(body=[Assign(targets=[Name(id='x', ctx=Store())], value=Str(s='hello'))])
>>> print(ast.dump(ast.parse("""
... x = "hello"
... """)))
Module(body=[Assign(targets=[Name(id='x', ctx=Store())], value=Str(s='hello'))])
>>> print(ast.dump(ast.parse("""
... x = "he" "ll" "o"
... """)))
Module(body=[Assign(targets=[Name(id='x', ctx=Store())], value=Str(s='hello'))])

Nothing in the expression evaluator knows or cares about what kind of
string literal you used, nor whether you included more than one. It's
all just alternative forms of string literal.

ChrisA

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