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# Conditional expressions - PEP 308

 P: n/a It would be helpful if the rules of the game were spelled out more clearly. The conditional expression is defined as X if C else Y. We don't know the precedence of the "if" operator. From the little test below, it seem to have a lower precedence than "or". Thus, it is desirable for the user to put the conditional expression in parentheses. Colin W. # condExpr.py # PEP 308 defines a conditional expression as X if C else Y # but we don't know exactly what X is supposed to be. # It doesn't seem to be spelled out in the syntax. def main(): names= ['abc', 'def', '_ghi', 'jkl', '_mno', 'pqrs'] res= '' for w in names: res= res + w if w[0] != '_' else '' z= 1 print 'res1:', res res= '' for w in names: res= res + (w if w[0] != '_' else '') z= 1 print 'res2:', res if __name__ == '__main__': main() Result: [Dbg]>>> res1: pqrs res2: abcdefjklpqrs Jan 30 '07 #1
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 P: n/a On Jan 30, 9:51 pm, "Colin J. Williams" >> res1: pqrs res2: abcdefjklpqrs But to give them credit though, in Whats new in Python 2.5: PEP 308, they do mention that as a matter of style you should parenthesise the if-expression, and the example given consistes of just a simple assignment of the if-expr to a name. - Paddy. Jan 31 '07 #2

 P: n/a Paddy wrote: On Jan 30, 9:51 pm, "Colin J. Williams" It would be helpful if the rules of the game were spelled out more clearly.The conditional expression is defined as X if C else Y.We don't know the precedence of the "if" operator. From the little testbelow, it seem to have a lower precedence than "or".Thus, it is desirable for the user to put the conditional expression inparentheses.Colin W.# condExpr.py# PEP 308 defines a conditional expression as X if C else Y# but we don't know exactly what X is supposed to be.# It doesn't seem to be spelled out in the syntax.def main(): names= ['abc', 'def', '_ghi', 'jkl', '_mno', 'pqrs'] res= '' for w in names: res= res + w if w[0] != '_' else '' z= 1 print 'res1:', res res= '' for w in names: res= res + (w if w[0] != '_' else '') z= 1 print 'res2:', resif __name__ == '__main__': main()Result:[Dbg]>>>res1: pqrsres2: abcdefjklpqrs But to give them credit though, in Whats new in Python 2.5: PEP 308, they do mention that as a matter of style you should parenthesise the if-expression, and the example given consistes of just a simple assignment of the if-expr to a name. - Paddy. Yes, I agree. The ternary operator is a step forward. I was trying to make the point that the parentheses are necessary if X is more than a simple value. It's a pity that one finds this out by experiment rather than definition. Colin W. Jan 31 '07 #3

 P: n/a Colin J. Williams wrote: It would be helpful if the rules of the game were spelled out more clearly. The conditional expression is defined as X if C else Y. We don't know the precedence of the "if" operator. From the little test below, it seem to have a lower precedence than "or". Thus, it is desirable for the user to put the conditional expression in parentheses. Could you submit a documentation patch? http://sourceforge.net/tracker/?grou...70&atid=105470 It doesn't need to be in LaTeX. Plain text is fine. Just indicate in what document you think text should be added. STeVe Jan 31 '07 #4

 P: n/a Colin J. Williams wrote: It would be helpful if the rules of the game were spelled out more clearly. The conditional expression is defined as X if C else Y. We don't know the precedence of the "if" operator. From the little test below, it seem to have a lower precedence than "or". The rules are specified in the Python Reference Manual: http://docs.python.org/ref/Booleans.html Ziga Jan 31 '07 #5

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