is compares identity. A string will never be identical to a not-string.
== is equality. But a string will never be equal to either True or False.
You want neither.
path = '/bla/bla/bla'
if path:
print "True"
else:
print "False"
(not True) vs (False)
boolean - 'True' and 'False' in Python - Stack Overflow
python - why is the expression "true or true and false" true? - Stack Overflow
python - Boolean identity == True vs is True - Stack Overflow
Videos
What is the difference? I still can’t get it? Both display false
is compares identity. A string will never be identical to a not-string.
== is equality. But a string will never be equal to either True or False.
You want neither.
path = '/bla/bla/bla'
if path:
print "True"
else:
print "False"
From 6.11. Boolean operations:
In the context of Boolean operations, and also when expressions are used by control flow statements, the following values are interpreted as false: False, None, numeric zero of all types, and empty strings and containers (including strings, tuples, lists, dictionaries, sets and frozensets). All other values are interpreted as true.
The key phrasing here that I think you are misunderstanding is "interpreted as false" or "interpreted as true". This does not mean that any of those values are identical to True or False, or even equal to True or False.
The expression '/bla/bla/bla' will be treated as true where a Boolean expression is expected (like in an if statement), but the expressions '/bla/bla/bla' is True and '/bla/bla/bla' == True will evaluate to False for the reasons in Ignacio's answer.
Since and has a higher precedence than or the expression
True or True and False
is evaluated as
True or (True and False)
which in turn evaluates to
True or False
which is straightforwardly True.
the notion true or true and false can be re-written as (true) or (true and false). and means both arguments should be true in order for the answer to be true, therefore, the (true and false) yields FALSE. or on the other hand means at least one of the arguments must be true. so true or false yields TRUE.
Watch more tutorial about this on youtube, there's a lot of helpful explanation there :)
If you want to determine whether a value is exactly True (not just a true-like value), is there any reason to use if foo == True rather than if foo is True?
If you want to make sure that foo really is a boolean and of value True, use the is operator.
Otherwise, if the type of foo implements its own __eq__() that returns a true-ish value when comparing to True, you might end up with an unexpected result.
As a rule of thumb, you should always use is with the built-in constants True, False and None.
Does this vary between implementations such as CPython (2.x and 3.x), Jython, PyPy, etc.?
In theory, is will be faster than == since the latter must honor types' custom __eq__ implementations, while is can directly compare object identities (e.g., memory addresses).
I don't know the source code of the various Python implementations by heart, but I assume that most of them can optimize that by using some internal flags for the existence of magic methods, so I suspect that you won't notice the speed difference in practice.
Never use is True in combination with numpy (and derivatives such as pandas):
In[1]: import numpy as np
In[2]: a = np.array([1, 2]).any()
In[4]: a is True
Out[4]: False
In[5]: a == True
Out[5]: True
This was unexpected to me as:
In[3]: a
Out[3]: True
I guess the explanation is given by:
In[6]: type(a)
Out[6]: numpy.bool_
if result is None:
print "error parsing stream"
elif result:
print "result pass"
else:
print "result fail"
keep it simple and explicit. You can of course pre-define a dictionary.
messages = {None: 'error', True: 'pass', False: 'fail'}
print messages[result]
If you plan on modifying your simulate function to include more return codes, maintaining this code might become a bit of an issue.
The simulate might also raise an exception on the parsing error, in which case you'd either would catch it here or let it propagate a level up and the printing bit would be reduced to a one-line if-else statement.
Don't fear the Exception! Having your program just log and continue is as easy as:
try:
result = simulate(open("myfile"))
except SimulationException as sim_exc:
print "error parsing stream", sim_exc
else:
if result:
print "result pass"
else:
print "result fail"
# execution continues from here, regardless of exception or not
And now you can have a much richer type of notification from the simulate method as to what exactly went wrong, in case you find error/no-error not to be informative enough.
First to answer your question, you set a variable to true or false by assigning True or False to it:
myFirstVar = True
myOtherVar = False
If you have a condition that is basically like this though:
if <condition>:
var = True
else:
var = False
then it is much easier to simply assign the result of the condition directly:
var = <condition>
In your case:
match_var = a == b
match_var = a==b
that should more than suffice
you cant use a - in a variable name as it thinks that is match (minus) var
match=1
var=2
print match-var #prints -1