Convert True/False value read from file to boolean

Use ast.literal_eval:

>>> import ast
>>> ast.literal_eval('True')
True
>>> ast.literal_eval('False')
False

Why is flag always converting to True?

Non-empty strings are always True in Python.

Related: Truth Value Testing

If NumPy is an option, then:

>>> import StringIO
>>> import numpy as np
>>> s = 'True - False - True'
>>> c = StringIO.StringIO(s)
>>> np.genfromtxt(c, delimiter='-', autostrip=True, dtype=None) #or dtype=bool
array([ True, False,  True], dtype=bool)

I’m not suggested this as the best answer, just an alternative but you can also do something like:

flag = reader[0] == "True"

flag will be True id reader[0] is “True”, otherwise it will be False.

You can use dict to convert string to boolean. Change this line flag = bool(reader[0]) to:

flag = {'True': True, 'False': False}.get(reader[0], False) # default is False

you can use distutils.util.strtobool

>>> from distutils.util import strtobool

>>> strtobool('True')
1
>>> strtobool('False')
0

True values are y, yes, t, true, on and 1; False values are n, no, f, false, off and 0. Raises ValueError if val is anything else.

Currently, it is evaluating to True because the variable has a value. There is a good example found here of what happens when you evaluate arbitrary types as a boolean.

In short, what you want to do is isolate the 'True' or 'False' string and run eval on it.

>>> eval('True')
True
>>> eval('False')
False

The cleanest solution that I’ve seen is:

from distutils.util import strtobool
def string_to_bool(string):
    return bool(strtobool(str(string)))

Sure, it requires an import, but it has proper error handling and requires very little code to be written (and tested).

pip install str2bool

>>> from str2bool import str2bool
>>> str2bool('Yes')
True
>>> str2bool('FaLsE')
False

If you want to be case-insensitive, you can just do:

b = True if bool_str.lower() == 'true' else False

Example usage:

>>> bool_str = 'False'
>>> b = True if bool_str.lower() == 'true' else False
>>> b
False
>>> bool_str = 'true'
>>> b = True if bool_str.lower() == 'true' else False
>>> b
True

If your data is from json, you can do that

import json

json.loads(‘true’)

True

You can do with json.

In [124]: import json

In [125]: json.loads('false')
Out[125]: False

In [126]: json.loads('true')
Out[126]: True

If you need quick way to convert strings into bools (that functions with most strings) try.

def conv2bool(arg):
   try:
     res= (arg[0].upper()) == "T"
   except Exception,e:
     res= False
   return res # or do some more processing with arg if res is false

Just to add that if your truth value can vary, for instance if it is an input from different programming languages or from different types, a more robust method would be:

flag = value in ['True','true',1,'T','t','1'] # this can be as long as you want to support

And a more performant variant would be (set lookup is O(1)):

TRUTHS = set(['True','true',1,'T','t','1'])
flag = value in truths

Using dicts to convert "True" in True:

def str_to_bool(s: str):
    status = {"True": True,
                "False": False}
    try:
        return status[s]
    except KeyError as e:
        #logging

If you have

>>> my_value = "False"

then either do

>>> my_value in "False"
True
>>> my_value in "True"
False

or

>>> "False" in my_value
True
>>> "True" in my_value
False

def strtobool(val):
    """Convert a string representation of truth to true (1) or false (0).
    True values are 'y', 'yes', 't', 'true', 'on', and '1'; false values
    are 'n', 'no', 'f', 'false', 'off', and '0'.  Raises ValueError if
    'val' is anything else.
    """
    val = val.lower()
    if val in ('y', 'yes', 't', 'true', 'on', '1'):
        return True
    elif val in ('n', 'no', 'f', 'false', 'off', '0'):
        return False
    else:
        raise ValueError("invalid truth value %r" % (val,))