[Python-bugs-list] [ python-Feature Requests-449187 ] if x in 1,2,3: should work

[SourceForge.net]

Feature Requests item #449187, was opened at 2001-08-08 11:07
Message generated for change (Comment added) made by rhettinger
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=355470&aid=449187&group_id=5470

Category: Parser/Compiler
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Alex Martelli (aleax)
Assigned to: Nobody/Anonymous (nobody)
Summary: if x in 1,2,3: should work

Initial Comment:
There is normally some symmetry between 'for x in 
whatever' and 'if x in whatever'.  However, the 
symmetry is broken (for syntax reasons I do not 
understand) in one case, a tuple-without-parentheses:
    for x in 1, 2, 3: pass
works fine, but
    if x in 1, 2, 3: pass
gives a syntax error.  Can the syntax error be 
removed?  I think it violates the principle of least 
astonishment, as one would expect that either both 
forms would work or neither would.
(I'd offer a patch, but I don't think I really 
understand the way Python syntax is implemented in the 
Python compiler...!-).


----------------------------------------------------------------------

>Comment By: Raymond Hettinger (rhettinger)
Date: 2003-05-12 23:04

Message:
Logged In: YES 
user_id=80475

No dice, then non-parenthesis version occurs throughout 
Python (blame the comma, not the parens):

t = 'a', 'b', 'c'  # This works.
return 'a', 'b', 'c'  # So does this.
for letter in 'a', 'b', 'c':  So this should also.



----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2003-05-12 20:53

Message:
Logged In: YES 
user_id=357491

That's why I want for to be changed so that it needs parentheses.

----------------------------------------------------------------------

Comment By: Tim Peters (tim_one)
Date: 2003-05-12 20:38

Message:
Logged In: YES 
user_id=31435

The fundamental problem is that, as an expression, "x in 1, 
2, 3" doesn't group as an if-statement would need it to 
group:

>>> y = 'a' in 'a', 'b', 'c'
>>> y
(True, 'b', 'c')
>>>

>>> for y in 'a' in 'a', 'b', 'c':
...     print y
...
True
b
c
>>>


So if Python did allow a testlist as the 'if' expression, it 
would be parsed as:

    if ((x in 1), 2, 3):

which is a surprising way of raising TypeError.  The parens 
are really needed to get the 1, 2, 3 part parsed as a tuple.

----------------------------------------------------------------------

Comment By: Brett Cannon (bcannon)
Date: 2003-05-12 20:11

Message:
Logged In: YES 
user_id=357491

I didn't know Python accepted that 'for' structure!?!  I agree with Alex that 
this should be changed at some point.

As for why this is, it appears that the offending grammar definitions are:

for_stmt: 'for' exprlist 'in' testlist ':' suite ['else' ':' suite]
if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]
testlist: test (',' test)* [',']
test: and_test ('or' and_test)* | lambdef

So it looks like the difference is 'for' uses testlist while 'if' uses just test.  
Probably can just swap out testlist for test and get the result you want.

----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=355470&aid=449187&group_id=5470