[Patches] [ python-Patches-471421 ] conditional expression (if-then-else)

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Patches item #471421, was opened at 2001-10-15 11:59
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Category: Core (C code)
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Guido van Rossum (gvanrossum)
Assigned to: Guido van Rossum (gvanrossum)
Summary: conditional expression (if-then-else)

Initial Comment:
Here's an implementation of conditional expressions of
the form

    if <test> then <test> else <test>

It's hairier than expected because I'm trying to
require you to put parentheses around it in some cases
but not in others.

This still lacking:

- a PEP to motivate and explain it
- documentation
- tests
- needed changes to Modules/parsermodule.c

After applying this patch, you must regenerate the
grammar; the Unix Makefile and the current CVS version
of the Windows project file do this automatically.

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>Comment By: Guido van Rossum (gvanrossum)
Date: 2001-10-15 12:47

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The 'fi' would disambiguate binary operators following the
else part. But I find it very unpythonic: we don't have
Algol-68 style reversed closing keywords anywhere else, and
when you squint you don't see the structure.

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Comment By: Tim Peters (tim_one)
Date: 2001-10-15 12:44

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Martin, I don't think so.  "The problem" is ensuring the 
parser can distinguish a conditional expression from an if 
statement, as they both begin with "if".  A left paren 
rules out an if-statement.  If the conditional expression 
*began* with "fi", no problem <wink>.

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Comment By: Martin v. Löwis (loewis)
Date: 2001-10-15 12:34

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Would the grammar become simpler if the syntax was

if <test> then <test> else <test> fi

?


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Comment By: Tim Peters (tim_one)
Date: 2001-10-15 12:22

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FYI, the grammar is not regenerated by magic on Windows.  I 
very recently checked in Parser/grammar.mak (a Windows 
nmake file), which you can use "by hand" (read the comments 
at the top of the file) to regenerate the grammar if you 
know that's needed.

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