问题
I tried this in python shell
>>> a='apple'
>>> b=11.1
>>> a>b
True
>>> a
'apple'
>>> b>a
False
Can someone explain to me how a>b is True? When a is an string and b is float.
回答1:
Order comparison between elements of different types was a "design bug" in Python 2.x that has been removed in Python 3 (where you get a runtime error).
Correcting it wasn't an option before 3.x because of a lot of existing software that does sorting on heterogeneous containers (and version 3 is the first in which backward compatibility is intentionally broken).
In Python 2.x you can compare anything for </>, with the only exception of complex numbers where this is explicitly forbidden.
The ordering result of comparison of different types is arbitrary but fixed at least for a given run of the Python interpreter, see http://docs.python.org/2/reference/expressions.html#not-in . Values of different non-numeric types are always considered different.
回答2:
In Python 2, comparison between incomparable types often "works", giving meaningless results. Sometimes it's fun: 'aleph0' > float('+inf'). But most of the time it's just silly.
So Python 3 has removed this, and 'a' > 1.0 quite reasonably raises a TypeError under it.
来源：https://stackoverflow.com/questions/19713498/compare-string-to-float-in-python