Converting from a string to boolean in Python

Question:

How do I convert a string into a boolean in Python? This attempt returns True:

>>> bool("False")
True
Asked By: Joan Venge

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Answers:

Really, you just compare the string to whatever you expect to accept as representing true, so you can do this:

s == 'True'

Or to checks against a whole bunch of values:

s.lower() in ['true', '1', 't', 'y', 'yes', 'yeah', 'yup', 'certainly', 'uh-huh']

Be cautious when using the following:

>>> bool("foo")
True
>>> bool("")
False

Empty strings evaluate to False, but everything else evaluates to True. So this should not be used for any kind of parsing purposes.

Answered By: Keith Gaughan

you could always do something like

my_string = "false"
val = (my_string == "true")

the bit in parens would evaluate to False. This is just another way to do it without having to do an actual function call.

Answered By: helloandre
def str2bool(v):
  return v.lower() in ("yes", "true", "t", "1")

Then call it like so:

>>> str2bool("yes")
True
>>> str2bool("no")
False
>>> str2bool("stuff")
False
>>> str2bool("1")
True
>>> str2bool("0")
False

Handling true and false explicitly:

You could also make your function explicitly check against a True list of words and a False list of words. Then if it is in neither list, you could throw an exception.

Answered By: Brian R. Bondy

Since Python 2.6 you can use ast.literal_eval:

>>> import ast
>>> help(ast.literal_eval)
Help on function literal_eval in module ast:

literal_eval(node_or_string)
    Safely evaluate an expression node or a string containing a Python
    expression.  The string or node provided may only consist of the following
    Python literal structures: strings, numbers, tuples, lists, dicts, booleans,
    and None.

Which seems to work, as long as you’re sure your strings are going to be either "True" or "False":

>>> ast.literal_eval("True")
True
>>> ast.literal_eval("False")
False
>>> ast.literal_eval("F")
Traceback (most recent call last):
  File "", line 1, in 
  File "/opt/Python-2.6.1/lib/python2.6/ast.py", line 68, in literal_eval
    return _convert(node_or_string)
  File "/opt/Python-2.6.1/lib/python2.6/ast.py", line 67, in _convert
    raise ValueError('malformed string')
ValueError: malformed string
>>> ast.literal_eval("'False'")
'False'

I wouldn’t normally recommend this, but it is completely built-in and could be the right thing depending on your requirements.

Answered By: Jacob Gabrielson

The usual rule for casting to a bool is that a few special literals (False, 0, 0.0, (), [], {}) are false and then everything else is true, so I recommend the following:

def boolify(val):
    if (isinstance(val, basestring) and bool(val)):
        return not val in ('False', '0', '0.0')
    else:
        return bool(val)
Answered By: Carl G

here’s a hairy, built in way to get many of the same answers. Note that although python considers "" to be false and all other strings to be true, TCL has a very different idea about things.

>>> import Tkinter
>>> tk = Tkinter.Tk()
>>> var = Tkinter.BooleanVar(tk)
>>> var.set("false")
>>> var.get()
False
>>> var.set("1")
>>> var.get()
True
>>> var.set("[exec 'rm -r /']")
>>> var.get()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/lib-tk/Tkinter.py", line 324, in get
    return self._tk.getboolean(self._tk.globalgetvar(self._name))
_tkinter.TclError: 0expected boolean value but got "[exec 'rm -r /']"
>>> 

A good thing about this is that it is fairly forgiving about the values you can use. It’s lazy about turning strings into values, and it’s hygenic about what it accepts and rejects(notice that if the above statement were given at a tcl prompt, it would erase the users hard disk).

the bad thing is that it requires that Tkinter be available, which is usually, but not universally true, and more significantly, requires that a Tk instance be created, which is comparatively heavy.

What is considered true or false depends on the behavior of the Tcl_GetBoolean, which considers 0, false, no and off to be false and 1, true, yes and on to be true, case insensitive. Any other string, including the empty string, cause an exception.

You probably already have a solution but for others who are looking for a method to convert a value to a boolean value using “standard” false values including None, [], {}, and “” in addition to false, no , and 0.

def toBoolean( val ):
    """ 
    Get the boolean value of the provided input.

        If the value is a boolean return the value.
        Otherwise check to see if the value is in 
        ["false", "f", "no", "n", "none", "0", "[]", "{}", "" ]
        and returns True if value is not in the list
    """

    if val is True or val is False:
        return val

    falseItems = ["false", "f", "no", "n", "none", "0", "[]", "{}", "" ]

    return not str( val ).strip().lower() in falseItems
Answered By: Chris McMillan
def str2bool(str):
  if isinstance(str, basestring) and str.lower() in ['0','false','no']:
    return False
  else:
    return bool(str)

idea: check if you want the string to be evaluated to False; otherwise bool() returns True for any non-empty string.

Answered By: xvga

Here’s something I threw together to evaluate the truthiness of a string:

def as_bool(val):
 if val:
  try:
   if not int(val): val=False
  except: pass
  try:
   if val.lower()=="false": val=False
  except: pass
 return bool(val)

more-or-less same results as using eval but safer.

Answered By: tylerl

Here’s is my version. It checks against both positive and negative values lists, raising an exception for unknown values. And it does not receive a string, but any type should do.

def to_bool(value):
    """
       Converts 'something' to boolean. Raises exception for invalid formats
           Possible True  values: 1, True, "1", "TRue", "yes", "y", "t"
           Possible False values: 0, False, None, [], {}, "", "0", "faLse", "no", "n", "f", 0.0, ...
    """
    if str(value).lower() in ("yes", "y", "true",  "t", "1"): return True
    if str(value).lower() in ("no",  "n", "false", "f", "0", "0.0", "", "none", "[]", "{}"): return False
    raise Exception('Invalid value for boolean conversion: ' + str(value))

Sample runs:

>>> to_bool(True)
True
>>> to_bool("tRUe")
True
>>> to_bool("1")
True
>>> to_bool(1)
True
>>> to_bool(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 9, in to_bool
Exception: Invalid value for boolean conversion: 2
>>> to_bool([])
False
>>> to_bool({})
False
>>> to_bool(None)
False
>>> to_bool("Wasssaaaaa")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 9, in to_bool
Exception: Invalid value for boolean conversion: Wasssaaaaa
>>>
Answered By: Petrucio

A dict (really, a defaultdict) gives you a pretty easy way to do this trick:

from collections import defaultdict
bool_mapping = defaultdict(bool) # Will give you False for non-found values
for val in ['True', 'yes', ...]:
    bool_mapping[val] = True

print(bool_mapping['True']) # True
print(bool_mapping['kitten']) # False

It’s really easy to tailor this method to the exact conversion behavior you want — you can fill it with allowed Truthy and Falsy values and let it raise an exception (or return None) when a value isn’t found, or default to True, or default to False, or whatever you want.

Answered By: Nate

This is the version I wrote. Combines several of the other solutions into one.

def to_bool(value):
    """
    Converts 'something' to boolean. Raises exception if it gets a string it doesn't handle.
    Case is ignored for strings. These string values are handled:
      True: 'True', "1", "TRue", "yes", "y", "t"
      False: "", "0", "faLse", "no", "n", "f"
    Non-string values are passed to bool.
    """
    if type(value) == type(''):
        if value.lower() in ("yes", "y", "true",  "t", "1"):
            return True
        if value.lower() in ("no",  "n", "false", "f", "0", ""):
            return False
        raise Exception('Invalid value for boolean conversion: ' + value)
    return bool(value)

If it gets a string it expects specific values, otherwise raises an Exception. If it doesn’t get a string, just lets the bool constructor figure it out. Tested these cases:

test_cases = [
    ('true', True),
    ('t', True),
    ('yes', True),
    ('y', True),
    ('1', True),
    ('false', False),
    ('f', False),
    ('no', False),
    ('n', False),
    ('0', False),
    ('', False),
    (1, True),
    (0, False),
    (1.0, True),
    (0.0, False),
    ([], False),
    ({}, False),
    ((), False),
    ([1], True),
    ({1:2}, True),
    ((1,), True),
    (None, False),
    (object(), True),
    ]
Answered By: Tom Ekberg

I like to use the ternary operator for this, since it’s a bit more succinct for something that feels like it shouldn’t be more than 1 line.

True if my_string=="True" else False
Answered By: Clayton Rabenda

I don’t agree with any solution here, as they are too permissive. This is not normally what you want when parsing a string.

So here the solution I’m using:

def to_bool(bool_str):
    """Parse the string and return the boolean value encoded or raise an exception"""
    if isinstance(bool_str, basestring) and bool_str: 
        if bool_str.lower() in ['true', 't', '1']: return True
        elif bool_str.lower() in ['false', 'f', '0']: return False

    #if here we couldn't parse it
    raise ValueError("%s is no recognized as a boolean value" % bool_str)

And the results:

>>> [to_bool(v) for v in ['true','t','1','F','FALSE','0']]
[True, True, True, False, False, False]
>>> to_bool("")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in to_bool
ValueError: '' is no recognized as a boolean value

Just to be clear because it looks as if my answer offended somebody somehow:

The point is that you don’t want to test for only one value and assume the other. I don’t think you always want to map Absolutely everything to the non parsed value. That produces error prone code.

So, if you know what you want code it in.

Answered By: estani

The JSON parser is also useful for in general converting strings to reasonable python types.

>>> import json
>>> json.loads("false".lower())
False
>>> json.loads("True".lower())
True
Answered By: Alan Marchiori

A cool, simple trick (based on what @Alan Marchiori posted), but using yaml:

import yaml

parsed = yaml.load("true")
print bool(parsed)

If this is too wide, it can be refined by testing the type result. If the yaml-returned type is a str, then it can’t be cast to any other type (that I can think of anyway), so you could handle that separately, or just let it be true.

I won’t make any guesses at speed, but since I am working with yaml data under Qt gui anyway, this has a nice symmetry.

Answered By: Rafe

I realize this is an old post, but some of the solutions require quite a bit of code, here’s what I ended up using:

def str2bool(value):
    return {"True": True, "true": True}.get(value, False)
Answered By: Ron E

Warning: This answer will no longer work as of Python 3.12 (it’s deprecated as of 3.10)

Use:

bool(distutils.util.strtobool(some_string))

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.

Be aware that distutils.util.strtobool() returns integer representations and thus it needs to be wrapped with bool() to get Boolean values.

Answered By: jzwiener

This version keeps the semantics of constructors like int(value) and provides an easy way to define acceptable string values.

def to_bool(value):
    valid = {'true': True, 't': True, '1': True,
             'false': False, 'f': False, '0': False,
             }   

    if isinstance(value, bool):
        return value

    if not isinstance(value, basestring):
        raise ValueError('invalid literal for boolean. Not a string.')

    lower_value = value.lower()
    if lower_value in valid:
        return valid[lower_value]
    else:
        raise ValueError('invalid literal for boolean: "%s"' % value)


# Test cases
assert to_bool('true'), '"true" is True' 
assert to_bool('True'), '"True" is True' 
assert to_bool('TRue'), '"TRue" is True' 
assert to_bool('TRUE'), '"TRUE" is True' 
assert to_bool('T'), '"T" is True' 
assert to_bool('t'), '"t" is True' 
assert to_bool('1'), '"1" is True' 
assert to_bool(True), 'True is True' 
assert to_bool(u'true'), 'unicode "true" is True'

assert to_bool('false') is False, '"false" is False' 
assert to_bool('False') is False, '"False" is False' 
assert to_bool('FAlse') is False, '"FAlse" is False' 
assert to_bool('FALSE') is False, '"FALSE" is False' 
assert to_bool('F') is False, '"F" is False' 
assert to_bool('f') is False, '"f" is False' 
assert to_bool('0') is False, '"0" is False' 
assert to_bool(False) is False, 'False is False'
assert to_bool(u'false') is False, 'unicode "false" is False'

# Expect ValueError to be raised for invalid parameter...
try:
    to_bool('')
    to_bool(12)
    to_bool([])
    to_bool('yes')
    to_bool('FOObar')
except ValueError, e:
    pass
Answered By: Michael Richmond

If you know that your input will be either "True" or something else, then why not use:

def bool_convert(s):
    return s == "True"
Answered By: Daniel van Flymen

I just had to do this… so maybe late to the party – but someone may find it useful

def str_to_bool(input, default):
    """
    | Default | not_default_str | input   | result
    | T       |  "false"        | "true"  |  T
    | T       |  "false"        | "false" |  F
    | F       |  "true"         | "true"  |  T
    | F       |  "true"         | "false" |  F

    """
    if default:
        not_default_str = "false"
    else:
        not_default_str = "true"

    if input.lower() == not_default_str:
        return not default
    else:
        return default
Answered By: Rcynic

By using Python’s built-in eval() function and the .capitalize() method, you can convert any “true” / “false” string (regardless of initial capitalization) to a true Python boolean.

For example:

true_false = "trUE"
type(true_false)

# OUTPUT: <type 'str'>

true_false = eval(true_false.capitalize())
type(true_false)

# OUTPUT: <type 'bool'>
Answered By: elPastor

If you have control over the entity that’s returning true/false, one option is to have it return 1/0 instead of true/false, then:

boolean_response = bool(int(response))

The extra cast to int handles responses from a network, which are always string.

Update 2021: "which are always string" — this is a naive observation. It depends on the serialization protocol used by the library. Default serialization of high-level libraries (the ones used by most web devs) is typically to convert to string before being serialized to bytes. And then on the other side, it’s deserialized from bytes to string, so you’ve lost any type information.

Answered By: Sam Malayek

If you know the string will be either "True" or "False", you could just use eval(s).

>>> eval("True")
True
>>> eval("False")
False

Only use this if you are sure of the contents of the string though, as it will throw an exception if the string does not contain valid Python, and will also execute code contained in the string.

Answered By: Joel Croteau

Yet another option

from ansible.module_utils.parsing.convert_bool import boolean
boolean('no')
# False
boolean('yEs')
# True
boolean('true')
# True
Answered By: Matt Kucia

Use package str2bool pip install str2bool

Answered By: Headmaster

If you like me just need boolean from variable which is string. You can use distils as mentioned by @jzwiener. However I could not import and use the module as he suggested.

Instead I end up using it this way on python3.7

distutils string to bool in python

from distutils import util # to handle str to bool conversion
enable_deletion = 'False'
enable_deletion = bool(util.strtobool(enable_deletion))

distutils is part of the python std lib, so no need to install anything, which is great!

Answered By: Gunay Anach

I use

# function
def to_bool(x):
    return x in ("True", "true", True)

# test cases
[[x, to_bool(x)] for x in [True, "True", "true", False, "False", "false", None, 1, 0, -1, 123]]
"""
Result:
[[True, True],
 ['True', True],
 ['true', True],
 [False, False],
 ['False', False],
 ['false', False],
 [None, False],
 [1, True],
 [0, False],
 [-1, False],
 [123, False]]
"""
Answered By: MrHIDEn

NOTE: DON’T EVER USE eval() if it takes an input directly or indirectly from the user because it is highly subject to abuse:

eval('os.system(‘rm -rf /’)')

But cheers! Study finds also that eval() is not evil and it is perfectly OK for TRUSTED CODE. You can use it to convert a boolean string such as "False" and "True" to a boolean type.


I would like to share my simple solution: use the eval(). It will convert the string True and False to proper boolean type IF the string is exactly in title format True or False always first letter capital or else the function will raise an error.

e.g.

>>> eval('False')
False

>>> eval('True')
True

Of course for dynamic variable you can simple use the .title() to format the boolean string.

>>> x = 'true'
>>> eval(x.title())
True

This will throw an error.

>>> eval('true')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name 'true' is not defined

>>> eval('false')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in <module>
NameError: name 'false' is not defined
Answered By: Roel

I was also required to change the input to bool for a function and the main input was only True or False in string. So, I just coded it like this:

def string_to_bool(s):
    bool_flag = True
    if s == "False":
        bool_flag = False
    elif s == "True":
        bool_flag = True
    else:
        print("Invalid Input")
    return bool_flag

You can also check it for more shortened for True and False like Y/N or y/n etc.

Answered By: ARHAM RUMI

You can also evaluate any string literal :

import ast
ast.literal_eval('True')  # True
type(ast.literal_eval('True'))  # <class 'bool'>


ls = '[1, 2, 3]'
ast.literal_eval(ls)  # [1, 2, 3]
type(ast.literal_eval(ls))  # <class 'list'>
Answered By: Nomi

I completely agree with the solution of @Jacob Gabrielson but the thing is ast.literal_eval only work with string value of True and False not with true or false. So you just have to use .title() for it to work

import ast
ast.literal_eval("false".title())
# or
ast.literal_eval("False".title())
Answered By: Anand Tripathi

By using below simple logic you can convert a string say a = 'true' or 'false', to boolean.

a = a.lower() == 'true'

if a == 'true' then this will set a=True and if a == 'false' then a=False.

Answered By: Sagar Deshmukh

There is an elegant solution with pydantic:

import pydantic

>>> pydantic.parse_obj_as(bool, "true")
True
Answered By: Yann

Use this solution:

def to_bool(value) -> bool:
    if value == 'true':
        return True
    elif value == 'True':
        return True
    elif value == 'false':
        return False
    elif value == 'False':
        return False
    elif value == 0:
        return False
    elif value == 1:
        return True
    else:
        raise ValueError("Value was not recognized as a valid Boolean.")
Answered By: srburton

we may need to catch ‘true’ case insensitive, if so:

>>> x="TrUE"  
>>> x.title() == 'True'  
True  

>>> x="false"  
>>> x.title() == 'True'  
False  

also note, it will return False for any other input which is neither true or false

Answered By: RE_Specto

In python version 3.10 you could do something like this;

def stringToBool(string: str) -> bool:
    match(string.lower()):
        case 'true':
            return True
        case 'false':
            return False

The match-statement is equivalent to switch in C++.

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