将从文件读取的 True/False 值转换为 boolean 值

我正在从一个文件中读取一个 True - False值，我需要将它转换为布尔值。目前，它总是将其转换为 True，即使值被设置为 False。

下面是我正在尝试做的 MWE:

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'

file.dat文件基本上由一个字符串组成，内部写有值 True或 False。这种排列看起来非常复杂，因为这是一个来自大得多的代码的最小示例，这是我如何将参数读入其中的。

为什么 flag总是转换成 True？

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最佳答案

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
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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)
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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.

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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
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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, y0, y1, y2 and y3. Raises y4 if y5 is anything else.

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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
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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).

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pip install str2bool

>>> from str2bool import str2bool
>>> str2bool('Yes')
True
>>> str2bool('FaLsE')
False
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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
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If your data is from json, you can do that

import json

json.loads('true')

True

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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
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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
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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
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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
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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
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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,))
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Unfortunately, strtobool is now deprecated.

Here is an implementation based on configparser which you can use instead of your bool:

import configparser


def strtobool(s):
try:
return configparser.ConfigParser.BOOLEAN_STATES[s.lower()]
except KeyError as e:
raise ValueError('Not a boolean: %s' % s) from e
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Ternary operator one-liner:

var_x = True if str_x == 'True' else False

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