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Hmm... An idea: if a,b==c,d:

John Hunter jdhunter at ace.bsd.uchicago.edu
Mon Nov 18 19:03:20 EST 2002 
>>>>> "Richard" == Richard Dillingham <shadowlord13_1 at yahoo.com> writes:

    Richard> As I have asked everyone else that brings up that point,
    Richard> then why does the following fail (Using Python 2.2.2)?

    if a,b==c,d: print a
      File "<input>", line 1
        if a,b==c,d: print a

To simplify note that the following also fails

if 1,2:
    pass

Now, I am no expert in the subtleties of the python language syntax,
but here is my read of the language reference (defns below from The
Python Reference Manual v2.2)


  The `if' statement
  ==================
  
     The `if' statement is used for conditional execution:
  
       if_stmt:        "if" expression ":" suite
                      ("elif" expression ":" suite)*
                      ["else" ":" suite]


The operative part of this is 'expression'.  


  Expression lists
  ================
  
       expression_list:      expression ("," expression)* [","]
  
   An expression list containing at least one comma yields a tuple.
  The length of the tuple is the number of expressions in the list.  The
  expressions are evaluated from left to right.


In the example
if 1,2:
  pass

1,2 is an expression list, *not* an expression


  Parenthesized forms
  -------------------
  
     A parenthesized form is an optional expression list enclosed in
  parentheses:
  
       parenth_form:      "(" [expression_list] ")"
  
     A parenthesized expression list yields whatever that expression list
  yields: if the list contains at least one comma, it yields a tuple;
  otherwise, it yields the single expression that makes up the expression
  list.

if (1,2):
  pass 

This is ok because (1,2) is a parenthesized form.  1,2 creates a
tuple, but from the language definition it is a expression_list, not
an expression.  (1,2) is an expression, so it can be used with the if
statement, while the expression list cannot.

Language mavens feel free to correct me.

John Hunter
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