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python-2.x

Why is an empty dictionary greater than 1?

Why is the following code true?

>>> foo = {}
>>> foo > 1
True
>>> foo < 1
False
>>> foo == 0
False
>>> foo == -1
False
>>> foo == 1
False

I understand what I wanted was len(foo) > 1, but as a beginner this surprised me.

Solution

  • From the docs:

    The operators <, >, ==, >=, <=, and != compare the values of two objects. The objects need not have the same type. If both are numbers, they are converted to a common type. Otherwise, objects of different types always compare unequal, and are ordered consistently but arbitrarily. You can control comparison behavior of objects of non-builtin types by defining a __cmp__ method or rich comparison methods like __gt__, described in section 3.4.

    (This unusual definition of comparison was used to simplify the definition of operations like sorting and the in and not in operators. In the future, the comparison rules for objects of different types are likely to change.)