In using a function, I wish to ensure that the type of the variables are as expected. How to do it right?
Here is an example fake function trying to do just this before going on with its role:
def my_print(begin, text, end):
"""Print 'text' in UPPER between 'begin' and 'end' in lower
"""
for i in (begin, text, end):
assert isinstance(i, str), "Input variables should be strings"
out = begin.lower() + text.upper() + end.lower()
print out
def test():
"""Put your test cases here!
"""
assert my_print("asdf", "fssfpoie", "fsodf")
assert not my_print("fasdf", 33, "adfas")
print "All tests passed"
test()
Is assert the right approach? Should I use try/except instead?
Also, my assert set of tests does not seem to work properly :S
Thanks pythoneers
The
isinstance
built-in is the preferred way if you really must, but even better is to remember Python's motto: "it's easier to ask forgiveness than permission"!-) (It was actually Grace Murray Hopper's favorite motto;-). I.e.:This, BTW, lets the function work just fine on Unicode strings -- without any extra effort!-)
You might want to try this example for version 2.6 of Python.
However, have you considered letting the function fail naturally instead?
Doing
type('')
is effectively equivalent tostr
andtypes.StringType
so
type('') == str == types.StringType
will evaluate to "True
"Note that Unicode strings which only contain ASCII will fail if checking types in this way, so you may want to do something like
assert type(s) in (str, unicode)
orassert isinstance(obj, basestring)
, the latter of which was suggested in the comments by 007Brendan and is probably preferred.isinstance()
is useful if you want to ask whether an object is an instance of a class, e.g:But for basic types, e.g.
str
,unicode
,int
,float
,long
etc askingtype(var) == TYPE
will work OK.