python - Convert True/False value read from file to boolean

Question

I'm reading a True - False value from a file and I need to convert it to boolean. Currently it always converts it to True even if the value is set to False.

Here's a MWE of what I'm trying to do:

with open('file.dat', mode="r") as f:
    for line in f:
        reader = line.split()
        # Convert to boolean <-- Not working?
        flag = bool(reader[0])

if flag:
    print 'flag == True'
else:
    print 'flag == False'

The file.dat file basically consists of a single string with the value True or False written inside. The arrangement looks very convoluted because this is a minimal example from a much larger code and this is how I read parameters into it.

Why is flag always converting to True?

Answer 1

bool('True') and bool('False') always return True because strings 'True' and 'False' are not empty.

To quote a great man (and Python documentation):

5.1. Truth Value Testing

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:

• …
• zero of any numeric type, for example, 0, 0L, 0.0, 0j.
• any empty sequence, for example, '', (), [].
• …

All other values are considered true — so objects of many types are always true.

The built-in bool function uses the standard truth testing procedure. That's why you're always getting True.

To convert a string to boolean you need to do something like this:

def str_to_bool(s):
    if s == 'True':
         return True
    elif s == 'False':
         return False
    else:
         raise ValueError # evil ValueError that doesn't tell you what the wrong value was