Why is this logical expression in python False?

My que开发者_运维知识库stion is, why are these expressions False?

Python 2.6.4 (r264:75706, Dec  7 2009, 18:45:15) 
[GCC 4.4.1] on linux2
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>>> num = raw_input("Choose a number: ")
Choose a number: 5
>>> print num
5
>>> print ( num < 18 )
False
>>> print ( num == 5 )
False

Because if i try this:

>>> print ( num > 0 )
True

The expression works fine.

This statement:

num = raw_input("Choose a number: ")

makes num a string, not a number, despite its misleading name. It so happens that Python 2 lets you compare strings with numbers, and in your version considers all strings larger than all numbers (the contents of the string play no role).

Use num = int(num) to make an integer (and be sure to use a try/except to catch possible errors when the user has typed something other than a number!) before you start comparing.

(In Python 3, the function's name changes from raw_input to input, and it still returns strings; however in Python 3 comparing a string with a number is considered an error, so you would get an exception rather than True or False in each of your comparison attempts).

The variable num does not actually contain the number 5; it contains the string "5". Because Python is strongly typed, 5 == "5" is False. Try converting it to an integer first:

>>> print (int(num) < 18)
True

num is a string. You can't meaningfully compare a string to an integer and a string is never equal to an integer (so == returns false and < and > return whatever they want). The reason that < and > don't throw an error (before python 3) when you compare strings and integers is to be able to sort heterogeneous lists.

Try num = float(raw_input("Choose..."))

You're evaluating a string in your boolean expressions.