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2009-12-03T14:42:41.733
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It's not so clear that a function must always return objects of a limited type, or that returning None is wrong. For instance, re.search can return a <code>_sre.SRE_Match</code> object or a <code>NoneType</code> object: <pre><code>import re match=re.search('a','a') type(match) # <type '_sre.SRE_Match'> match=re.search('a','b') type(match) # <type 'NoneType'> </code></pre> Designed this way, you can test for a match with the idiom <pre><code>if match: # do xyz </code></pre> If the developers had required re.search to return a <code>_sre.SRE_Match</code> object, then the idiom would have to change to <pre><code>if match.group(1) is None: # do xyz </code></pre> There would not be any major gain by requiring re.search to always return a <code>_sre.SRE_Match</code> object. So I think how you design the function must depend on the situation and in particular, how you plan to use the function. Also note that both <code>_sre.SRE_Match</code> and <code>NoneType</code> are instances of object, so in a broad sense they are of the same type. So the rule that "functions should always return only one type" is rather meaningless. Having said that, there is a beautiful simplicity to functions that return objects which all share the same properties. (Duck typing, not static typing, is the python way!) It can allow you to chain together functions: foo(bar(baz))) and know with certainty the type of object you'll receive at the other end. This can help you check the correctness of your code. By requiring that a function returns only objects of a certain limited type, there are fewer cases to check. "foo always returns an integer, so as long as an integer is expected everywhere I use foo, I'm golden..."
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1. POWhy should functions always return the same type?
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