5/18/2008
Python ternary operator
Today’s Python discovery:
Python doesn’t have the C style ?: ternary operator (e.g.: cond ? valueIfTrue : valueIfFalse).
But as of Python 2.5 it has a ternary operator with its own syntax: value_when_true if condition else value_when_false
For example:
>>> 'a' if 1 == 1 else 'b' 'a' >>> 'a' if 1 == 0 else 'b' 'b'
This is actually clearer and more Pythonic than that ?:
Unfortunately, for Python versions < 2.5, you don’t have this. I’ve seen people use: (condition and [value_when_true] or [value_when_false])[0]
IMHO, this is clever – in a bad way. Yuck. Personally, I think I’d rather just do:
def if_cond_val1_else_val2(cond, val1, val2): if cond: return val1 else: return val2
This adds 3 lines to your program (or 1 if you stick it in a module that you import from your programs) and won’t cause your colleagues to hate you.
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Your ifelse function is substantially different when it comes to evaluating val1 and val2. Both of them are evaluated no matter what, which might be expensive.
The and/or hack has almost-correct evaluation, but with the flaw that [value when false] gets chosen if [value when true] happens to be itself false. The other py2.4- version is like this:
print “%d %s” % (x, ['egg', 'eggs'][x > 1])
Too much evaluation, but mostly readable and without the subtle flaw of the and/or version.
Oops- not “x > 1″ but “x != 1″. That way we can have “0 eggs”.
I think the ternary operator evaluation is better translated by this:
eggs = 10
result = (”not so many”,”more than a dozen”)[eggs>12]
print result
a little weird, but cleaner. i guess it is more natural to write tuples in pythonese…
The >> True and ‘a’ or ‘b’
‘a’
>>> False and ‘a’ or ‘b’
‘b’
To drewp:
“The and/or hack has almost-correct evaluation, but with the flaw that [value when false] gets chosen if [value when true] happens to be itself false.”
If I understand your comment, it is inexact:
[False] is True because it is a non-empty sequence
(True and [False] or [22])[0]
Out[1]: False
This solution, albeit unreadable, is better than the other solutions because the [value when true] and [value when false]
are only evaluated if the condition applies.
In [5]: def p(x):
print x
return x
…:
In [8]: (True and [p(10)] or [p(22)])[0]
10
Out[8]: 10
In [9]: [p(22), p(10)][True]
22
10
Out[9]: 10
Bye,
Luca