2014-06-09 23:34 GMT+08:00 Roy Smith <[email protected]>: > We noticed recently that: > > >>> None in 'foo' > > raises (at least in Python 2.7) > > TypeError: 'in <string>' requires string as left operand, not NoneType > > This is surprising. The description of the 'in' operatator is, 'True if > an item of s is equal to x, else False '. From that, I would assume it > behaves as if it were written: > > for item in iterable: > if item == x: > return True > else: > return False > > why the extra type check for str.__contains__()? That seems very > unpythonic. Duck typing, and all that. >
It's a little bit inconsistent. But it's clearly documented here: https://docs.python.org/3/reference/expressions.html#in Which, according to its own logic, the string is not a *container* type. It's just some chars, and that totally makes sense for to restrict the type of x in "str" to be convertible to type str. On the other hand, containers like list, and tuple, they are heterogeneous by default in Python, so a item by item comparison is needed. -- 吾輩は猫である。ホームーページはhttp://introo.me。
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