python: replace elements in list with conditional

Question

I am trying to do the following with python and am having a strange behavior. Say I have the following list:

x = [5, 4, 3, 2, 1]

Now, I am doing something like:

x[x >= 3] = 3

This gives:

x = [5, 3, 3, 2, 1]

Why does only the second element get changed? I was expecting:

[3, 3, 3, 2, 1]

Answer 1

Because Python will evaluated the x>=3 as True and since True is equal to 1 so the second element of x will be converted to 3.

For such purpose you need to use a list comprehension :

>>> [3 if i >=3 else i for i in x]
[3, 3, 3, 2, 1]

And if you want to know that why x >= 3 evaluates as True, see the following documentation :

CPython implementation detail: Objects of different types except numbers are ordered by their type names; objects of the same types that don’t support proper comparison are ordered by their address.

In python-2.x and CPython implementation of course, a list is always greater than an integer type.As a string is greater than a list :

>>> ''>[]
True

In Python-3.X, however, you can't compare unorderable types together and you'll get a TypeError in result.

In [17]: '' > []
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-17-052e7eb2f6e9> in <module>()
----> 1 '' > []

TypeError: unorderable types: str() > list()