[issue29561] Interactive mode gives sys.ps2 not sys.ps1 after comment-only line

[Eryk Sun]

Eryk Sun  added the comment:

The is fixed in 3.7+.

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resolution:  -> out of date
stage:  -> resolved
status: open -> closed

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[issue29561] Interactive mode gives sys.ps2 not sys.ps1 after comment-only line

[Terry J. Reedy]

Terry J. Reedy added the comment:

If src is effectively blank, compile(src, filename, mode) raises SyntaxError if 
mode is 'single' but not if it is 'exec'.  I believe IDLE compiles with 
'single', but it has the behavior Jim (and I) expect and consider correct, 
printing '>>>' after effectively blank lines.  This is because IDLE uses 
code.InteractiveInterpreter, which uses codeop.compile_command, which uses 
codeop._maybe_compile, which replaces effectively blank statements with 'pass'. 
 compile('pass', '', 'single') returns a do-nothing code object.  The C-coded 
interactive interpreter is doing something else.

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nosy: +terry.reedy

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[issue29561] Interactive mode gives sys.ps2 not sys.ps1 after comment-only line

[Jim DeLaHunt]

Jim DeLaHunt added the comment:

Thank you for the analysis, Eryk Sun.

You wrote, "the interpreter is correctly conveying that it's still tokenizing 
the input; it hasn't compiled or executed any code." I think one important 
question for this issue is, what meaning should the choice of prompt convey?  
That breaks down into, 1. what meaning does the user believe it conveys, and 2. 
what meaning does the present behaviour of the code imply?

"it's still tokenizing the input" translates to a meaning, sys.ps1 indicates 
the interpreter just finished executing and has no input, while sys.ps2 
indicates the interpreter is tokenizing input and awaiting a chance to execute.

I think my intuition is, and the bash shell's behaviour says, sys.ps1 indicates 
the interpreter is ready to start a new statement, and sys.ps2 indicates the 
interpreter is in the midst of a compound statement.

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[issue29561] Interactive mode gives sys.ps2 not sys.ps1 after comment-only line

[Eryk Sun]

Eryk Sun added the comment:

For the tokenizer, a blank line is "[a] logical line that contains only spaces, 
tabs, formfeeds and possibly a comment" [1]. A blank line is normally ignored, 
except in the REPL an entirely blank line (i.e. no whitespace or comment) is 
used to end a multi-line statement.

This behavior is coded in Parser/tokenizer.c in tok_get(). After removing 
leading whitespace to get the indentation level, it decides whether the line 
should be ignored as blank as follows:

if (c == '#' || c == '\n') {
/* Lines with only whitespace and/or comments
   shouldn't affect the indentation and are
   not passed to the parser as NEWLINE tokens,
   except *totally* empty lines in interactive
   mode, which signal the end of a command group. */
if (col == 0 && c == '\n' && tok->prompt != NULL) {
blankline = 0; /* Let it through */
}
else {
blankline = 1; /* Ignore completely */
}
/* We can't jump back right here since we still
   may need to skip to the end of a comment */
}

The tokenizer switches to the ps2 prompt unconditionally, even if the first 
line was ignored as a blank line. One can argue that this behavior is outside 
of the norm for a shell or REPL. For example, bash doesn't switch to its PS2 
prompt after ignoring an initial blank line. On the other hand, the interpreter 
is correctly conveying that it's still tokenizing the input; it hasn't compiled 
or executed any code.

[1]: https://docs.python.org/3/reference/lexical_analysis.html#blank-lines

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[issue29561] Interactive mode gives sys.ps2 not sys.ps1 after comment-only line

[Jim DeLaHunt]

New submission from Jim DeLaHunt:

When you run the Python interpreter in interactive mode, get a sys.ps1 prompt 
(`...`), and type a comment-only or white-space-only line, the interpreter 
responds with a sys.ps2 prompt (`...`), instead of a sys.ps1 prompt. This seems 
wrong.

For example: 

% ./python.exe 
Python 3.7.0a0 (default, Feb 13 2017, 16:27:07) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> # comment-only<<-- line A
...   <<-- line B
>>>   <<-- line C 
>>>   <<-- line D 

Line A is a comment-only line entered by the user. The interpreter responds 
with the sys.ps2 prompt (`...`), in line B. Line C is a null line (user enters 
only the Enter key, nothing else). The interpreter responds with a sys.ps1 
prompt (`...`).  

If user enters a white-space-only line at a sys.ps1 prompt, the interpreter 
responds the same as if it were a comment-only line, with a sys.ps2 prompt 
(line B).

The expected expected interpreter behaviour in response to a  comment-only line 
or a white-space-only line, not in the midst of a compound statement, is that 
it be the same as to a null line, with a sys.ps1 prompt. 

In the context of a compound statement, the expected interpreter behaviour is 
that a comment-only line or a white-space-only line continue the compound 
statement. Entering a null line in a compound statement ends the compound 
statement.

Another way to phrase it: the expected interpreter behaviour is that a 
comment-only line or white-space-only line not change the interpreter prompt: 
if it was sys.ps1, it remains so; if it was sys.ps2, it remains so.

I have reproduced this behaviour on my build of cpython 3.7.a0, python 3.6 from 
MacPorts, python 2.7 from MacPorts, all on Mac OS X.  I see the same behaviour 
in the interactive shell at python.org/shell .

This makes me suspect that the Python design says my expectations are wrong. In 
that case, the current behaviour certainly is surprising. It should be 
documented.  I haven't found such documentation yet.

--
components: Interpreter Core
messages: 287799
nosy: JDLH
priority: normal
severity: normal
status: open
title: Interactive mode gives sys.ps2 not sys.ps1 after comment-only line
type: behavior
versions: Python 2.7, Python 3.5, Python 3.6, Python 3.7

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