Confusing SyntaxError while entering non-indented code in interactive interpreter on continuation line.

Patrick Maupin pmaupin at gmail.com
Sun Apr 25 22:45:06 CEST 2010


On Apr 25, 3:31 pm, Colin Howell <colin.d.how... at gmail.com> wrote:
> [I originally sent this to python-help; the volunteer who responded
> thought it was OK to repost it here.]
>
> I'm sure this has been discussed somewhere before, but I can't find it
> in the Python issue tracker. The following behavior from the
> interactive interpreter is rather confusing. (I've seen this behavior
> both under Python 2.6.5 on 32-bit Windows XP, installed from the
> standard Windows binary installer available from python.org, and under
> Python 2.4.3 on a Fedora Core 4 32-bit x86 Linux system.)
>
> The following do-nothing code is valid Python:
>
> if True:
>     pass
> pass
>
> A script file containing it will execute without error when passed to
> the Python interpreter from the command line, or when run from an IDLE
> edit window using the Run Module command (F5) key.
>
> However, if the interpreter is run in interactive mode from the
> command line, and I attempt to enter the same code from the
> interactive prompt as follows, I get a SyntaxError at the outer "pass"
> statement:
>
> >>> if True:
>
> ...     pass
> ... pass
>  File "<stdin>", line 3
>    pass
>       ^
> SyntaxError: invalid syntax
>
> The inner "pass" statement can be indented using spaces or with the
> Tab key; it doesn't matter. The problem is that I entered the outer
> "pass" statement on the second continuation line, rather than simply
> hitting return and waiting for a new primary prompt. But since the
> second continuation line is not indented unless you enter the
> indentation yourself, this is actually an easy mistake to make. One
> might think the parser will recognize that the inner block's
> indentation has been removed on the new line and that the line
> therefore represents a new statement.
>
> The same thing can happen in IDLE, except that IDLE automatically
> indents the continuation line and doesn't print a secondary prompt.
> But if you delete IDLE's indentation on the next continuation line and
> enter a new statement, you get the same SyntaxError as described
> above.
>
> What led me to this behavior? Example 6.1 on the following page from
> Dive Into Python:
>
> http://diveintopython.org/file_handling/index.html
>
> In this case, the code is a try-except statement, followed by a print.
> Note that the print is entered on a continuation line, which led me to
> carelessly try the same thing and then puzzle about it for quite a
> while before it finally hit me what I had been doing.
>
> I know that Dive Into Python is quite old and there have been many
> improvements in the language since, but I still think this gotcha
> needs to be looked at, at least to add a warning about it in the FAQ
> or tutorial, or to make the nature of the syntax error more obvious.

I agree that this is an easy mistake to make.  It's particularly easy
to do when cutting and pasting into an interactive shell.  I don't
think you could call this an error on the part of the python
interpreter, but it seems like it would be a great feature enhancement
to make the interpreter work more consistently.  After all, you can
happily unindent to any level except the far left:

>>> if a:
...   if b:
...     c
...   d
... e
  File "<stdin>", line 5
    e
    ^
SyntaxError: invalid syntax

Regards,
Pat



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