在Python中从字符串转换为布尔值？

Does anyone know how to do convert from a string to a boolean in Python? I found this link. But it doesn’t look like a proper way to do it. I.e. using built-in functionality, etc.

The reason I’m asking this is because I learned about int("string") from here. But when trying bool("string") it always returns True:

>>> bool("False")
True

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.

Use:

bool(distutils.util.strtobool(some_string))

• Python 2: http://docs.python.org/2/distutils/apiref.html?highlight=distutils.util#distutils.util.strtobool
• Python 3: https://docs.python.org/3/distutils/apiref.html#distutils.util.strtobool

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.

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.

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

Starting with Python 2.6, there is now 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.

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.

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

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
>>>

you could always do something like

myString = "false"
val = (myString == "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.

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.

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.

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.

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

You can simply use the built-in function eval():

a='True'
if a is True:
    print 'a is True, a type is', type(a)
else:
    print "a isn't True, a type is", type(a)
b = eval(a)
if b is True:
    print 'b is True, b type is', type(b)
else:
    print "b isn't True, b type is", type(b)

and the output:

a isn't True, a type is <type 'str'>
b is True, b type is <type 'bool'>

Yet another option

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

But in production if you don’t need ansible and all its dependencies, a good idea is to look at its source code and copy part of the logic that you need.

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)

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),
    ]

If you know that your input will be either “True” or “False” then why not use:

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

I use

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

# test cases
[[x, toBool(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]]
"""

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 myString=="True" else False

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)

Use package str2bool pip install str2bool

If you like me just need boolean from variable which is string. You can use distils as mentioned earlier 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 of installation. Which is great!👍

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

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.

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.

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.

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

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.

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'>