New Post has been published on ClintButler.net
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Python Tricks: "for" (and "while") loops can have an "else" branch?!
# Python's `for` and `while` loops # support an `else` clause that executes # only if the loops terminates without # hitting a `break` statement. def contains(haystack, needle): """ Throw a ValueError if `needle` not in `haystack`. """ for item in haystack: if item == needle: break else: # The `else` here is a # "completion clause" that runs # only if the loop ran to completion # without hitting a `break` statement. raise ValueError('Needle not found') >>> contains([23, 'needle', 0xbadc0ffee], 'needle') None >>> contains([23, 42, 0xbadc0ffee], 'needle') ValueError: "Needle not found" # Personally, I'm not a fan of the `else` # "completion clause" in loops because # I find it confusing. I'd rather do # something like this: def better_contains(haystack, needle): for item in haystack: if item == needle: return raise ValueError('Needle not found') # Note: Typically you'd write something # like this to do a membership test, # which is much more Pythonic: if needle not in haystack: raise ValueError('Needle not found')