On Tue, Apr 3, 2018 at 7:34 PM Ethan Furman <et...@stoneleaf.us> wrote:
> This behavior was recently brought to my attention [1]: > > --> 1 in 'hello' > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: 'in <string>' requires string as left operand, not int > > However, in any other collection (set, dict, list, tuple, etc), the answer > would be False. > > Does anyone remember the reason why an exception is raised in the string > instance instead of returning False? > If I had to hazard a guess, I'd say it's because strings by definition, in the sense that they are a container, can only contain strings of length 1, whereas the other containers you listed can contain a wide variety of Python objects (in some cases, any Python object). Thus it's easy to detect and raise the TypeError. Note that `[] in set()` also raises TypeError, so it's consistent: values of types that are impossible for the container to contain generally raise TypeError (in this case, an unhashable type and a container that only accepts hashable types). (I may be oversimplifying this a bit, but I think the basic idea is right.)
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