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

Answer 1

After having seen so many home brewn solutions, why not use something existing? All you need is:

from attrs.converters import to_bool

From the docs:

Convert "boolean" strings (e.g., from env. vars.) to real booleans

It does what you want: convert the strings True, true, yes, on, 1 (and some more) to True and False, false etc... to False.

It also raises an error if the string does not represent something which can be interpreted as boolean.

attrs is available from pypi.

Answer 2

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"""
    # replace basestring by str in python 3.x
    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.

Answer 3

If you want to be able to customize it, while having appropriate defaults for it:

def tobool(val, truthy=["true", "1", "on", "y", "yes", "t", "i"], falsy=["false", "0", "off", "n", "no", "f", "o"], strfunc = (lambda x: str(x).lower())):
    ch = strfunc(val)
    if ch in truthy:
        return True
    elif ch in falsy:
        return False
    else:
        return None

Answer 4

simple solution is

RUN_LOCAL = True if os.environ.get("RUN_LOCAL") == "True" else False

In this case if RUN_LOCAL variable is "True" it will be converted to boolean True in RUN_LOCAL variable else it will be False. You can add additional verifications if you are planning to input other strings like low case true and false and so on but the idea is simple and works without a need of creating new functions

Answer 5

There is an elegant solution with pydantic:

For pydantic >=2:

from pydantic import TypeAdapter
    
>>> TypeAdapter(bool).validate_python("true")
True

>>> TypeAdapter(bool).validate_python("off")
False

For pydantic <2:

import pydantic
    
>>> pydantic.parse_obj_as(bool, "true")
True

>>> pydantic.parse_obj_as(bool, "off")
False

Answer 6

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

valid = {'true': True, 't': True, '1': True,
         'false': False, 'f': False, '0': False,
         }

def to_bool(value):
    """Convert string value to boolean."""

    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

Answer 7

WARNING: Do not use the following code unless you actually know what you are doing with it. Please read the attached disclaimers and make sure you trust your inputs as using this on untrusted inputs could destroy your data and/or cost you your job.

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.

Answer 8

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))

• Python 2: distutils.util.strtobool
• Python >=3, <3.12: distutils.util.strtobool
• Python >=3.12: No longer part of the standard library due to PEP 632 – Deprecate distutils module

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.

Given that distutils will no longer be part of the standard library, here is the code for distutils.util.strtobool() (see the source code for 3.11.2).

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 1
    elif val in ('n', 'no', 'f', 'false', 'off', '0'):
        return 0
    else:
        raise ValueError("invalid truth value %r" % (val,))

Answer 9

Since Python 2.6 you can use ast.literal_eval, and it's still available in Python 3.

Evaluate an expression node or a string containing only a Python literal or container display. The string or node provided may only consist of the following Python literal structures: strings, bytes, numbers, tuples, lists, dicts, sets, booleans, None and Ellipsis.

This can be used for evaluating strings containing Python values without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing.

This function had been documented as “safe” in the past without defining what that meant. That was misleading. This is specifically designed not to execute Python code, unlike the more general eval(). There is no namespace, no name lookups, or ability to call out. But it is not free from attack: A relatively small input can lead to memory exhaustion or to C stack exhaustion, crashing the process. There is also the possibility for excessive CPU consumption denial of service on some inputs. Calling it on untrusted data is thus not recommended.

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 "<stdin>", line 1, in <module>
  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.

Answer 10

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

If you only want to map known values and throw an exception otherwise:

truthy_strings = ['True', 'yes']  # ... and so on
falsy_strings = ['False', 'no']   # ... and so on

bool_mapping = {}
for v in truthy_strings:
    bool_mapping[v] = True
for v in falsy_strings:
    bool_mapping[v] = False

If you want to be too clever by half, you can shorten this with itertools

from itertools import chain, repeat

bool_mapping = dict(chain(zip(truthy_strings, repeat(True)), zip(falsy_strings, repeat(False))))

This is probably dumb but itertools tricks are vaguely amusing.

Answer 11

This is an answer that uses code from the Django Rest Framework (DRF) 3.14.

You can either:

from rest_framework.fields import BooleanField
f = BooleanField(allow_null=True)
test_values = [ True, "True", "1", 1, -1, 1.0, "true", "t", "on",
         None, "null", "NULL",
         False, "False", "0", 0, "false", "f", 0.0, "off" ]
for item in test_values:
    r = f.to_internal_value(item)
    print(r)
   
# a shorter version
from rest_framework.fields import BooleanField
test_values = [ True, "True", "1", 1, -1, 1.0, "true", "t", "on",
         None, "null", "NULL",
         False, "False", "0", 0, "false", "f", 0.0, "off" ]
for item in test_values:
    print(BooleanField(allow_null=True).to_internal_value(item))

Or you could adapt the code of the BooleanField so that it suits your need. Here is the actual code of the class BooleanField as in DRF 3.x

# from rest_framework.fields 
# ...

class BooleanField(Field):
    default_error_messages = {
        'invalid': _('Must be a valid boolean.')
    }
    default_empty_html = False
    initial = False
    TRUE_VALUES = {
        't', 'T',
        'y', 'Y', 'yes', 'Yes', 'YES',
        'true', 'True', 'TRUE',
        'on', 'On', 'ON',
        '1', 1,
        True
    }
    FALSE_VALUES = {
        'f', 'F',
        'n', 'N', 'no', 'No', 'NO',
        'false', 'False', 'FALSE',
        'off', 'Off', 'OFF',
        '0', 0, 0.0,
        False
    }
    NULL_VALUES = {'null', 'Null', 'NULL', '', None}
    
    def to_internal_value(self, data):
        try:
            if data in self.TRUE_VALUES:
                return True
            elif data in self.FALSE_VALUES:
                return False
            elif data in self.NULL_VALUES and self.allow_null:
                return None
        except TypeError:  # Input is an unhashable type
            pass
        self.fail('invalid', input=data)
    
    def to_representation(self, value):
        if value in self.TRUE_VALUES:
            return True
        elif value in self.FALSE_VALUES:
            return False
        if value in self.NULL_VALUES and self.allow_null:
            return None
        return bool(value)

# ...

Answer 12

The top-rated answer is fine for limited cases or situations where you can make strong assumptions about the data you are processing. However, because custom objects can override __eq__ equality checking in Python, there is a significant pitfall. Consider the deliberately over-simplified toy example below:

In [1]: class MyString: 
   ...:     def __init__(self, value): 
   ...:         self.value = value 
   ...:     def __eq__ (self, obj): 
   ...:         if hasattr(obj, 'value'): 
   ...:             return obj.value == self.value 
   ...:         return False 
   ...:                                                                                                                                           

In [2]: v = MyString("True")                                                                                                                      

In [3]: v == "True"                                                                                                                               
Out[3]: False

If you imagine someone inheriting from a string type for MyString or implementing all kinds of native string methods, repr, etc., so that MyString instances mostly behave exactly like strings, but have the special extra value step in equality checking, then simple use of == 'True' would fail, and most likely it would be a silent failure from the user's perspective.

This is why it's good practice to coerce type into the exact nature of equality checking you want to perform, put that encapsulated into a helper function, and be pedantic about relying on that kind of "registered" way to validate things. For example with MyString you might write something like this,

def validate(s):
    if isinstance(s, str):
        return s == 'True'
    elif isinstance(s, MyString):
        return s.value == 'True' # <-- business logic
    ...
    raise ValueError(f"Type {type(s)} not supported for validation.")

Or another often used pattern is the reverse perspective where you define exactly one behavior for validation but you have a helper function that forces coercion into a type amenable for that single validation behavior, such as

def to_str(s):
    if isinstance(s, str):
        return s
    elif isinstance(s, MyString):
        return s.value
    ...
    raise ValueError(f"Unsupported type {type(s)}")

def validate(s):
    return to_str(s) == 'True'
    

It might look like we're adding a lot of boilerplate and verbosity. We could glibly express critique by saying, "why write all that if you can just write s == 'True'?" - But it misses the point that when you are validating something, you need to make sure all of your preconditions hold for the validation logic to be applied. If you can assume some data is a plain str type and you don't need to do any of that precondition (such as type) checking, great - but that's a very rare situation and it can be misleading to characterize the general situation for this question as being amenable to one super short and concise equality check.

Answer 13

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

Answer 14

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

def bool_convert(s):
    return s == "True"

Answer 15

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++.

Answer 16

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

Answer 17

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.")

Answer 18

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

Answer 19

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.

Answer 20

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

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! 👍

Answer 21

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.

Answer 22

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]]
"""

Answer 23

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.

Answer 24

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())

Answer 25

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'>

Answer 26

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.

Answer 27

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.

Answer 28

Yet another option

from ansible.module_utils.parsing.convert_bool import boolean
boolean('no')
# False
boolean('yEs')
# True
boolean('true')
# True

Answer 29

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.

Answer 30

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.