Re: verify the return value of a function
Jabba Laci wrote: Hi, In a unit test, I want to verify that a function returns a cookielib.LWPCookieJar object. What is the correct way of doing that? 1) First I tried to figure out its type with type(return_value) but it is type 'instance' 2) return_value.__class__ .__name__ gives 'LWPCookieJar', which is bettter 3) isinstance(return_value, cookielib.LWPCookieJar) seems to be the best way, however somewhere I read that using isinstance is discouraged Thanks, Laszlo isinstance is fine, if you could find the source where it is discouraged... Could be a consequence of some specific context. However, checking types in OOP is in general a failure. Unitary tests are possibly an exception. JM -- http://mail.python.org/mailman/listinfo/python-list
Re: verify the return value of a function
Am 19.01.2012 21:45, schrieb Jabba Laci: In a unit test, I want to verify that a function returns a cookielib.LWPCookieJar object. What is the correct way of doing that? 1) First I tried to figure out its type with type(return_value) but it istype 'instance' I'm not sure where the problem here is and where exactly you are seeing this. This might even indicate a problem with how the returned type is constructed. Anyhow: x = 1 type(x) type 'int' type(x) is int True So checking for an exact type should work using type(). 2) return_value.__class__ .__name__ gives 'LWPCookieJar', which is bettter It doesn't cover namespaces though. Also, you should compare that to cookielib.LWPCookieJar.__name__, not 'LWPCookieJar'. What is the LWP, btw? 3) isinstance(return_value, cookielib.LWPCookieJar) seems to be the best way, however somewhere I read that using isinstance is discouraged. Never trust any such claim that doesn't give a justification. In your case, that would be the right thing to do, IMHO. Promising to return an LWPCookieJar is fulfilled when the returnvalue is of that type or a class derived from that, which variant 1 doesn't cover. Uli -- http://mail.python.org/mailman/listinfo/python-list
Re: verify the return value of a function
Jean-Michel Pichavant wrote: isinstance is fine, if you could find the source where it is discouraged... Could be a consequence of some specific context. However, checking types in OOP is in general a failure. Unitary tests are possibly an exception. I think it's discouraged when people try to write big overloaded functions that check the types of the arguments to decide what they should be doing. In diagnostics and tests like the OP's there should be no problem. Mel. -- http://mail.python.org/mailman/listinfo/python-list
Re: verify the return value of a function
In article mailman.4872.1327005963.27778.python-l...@python.org, Jabba Laci jabba.l...@gmail.com wrote: Hi, In a unit test, I want to verify that a function returns a cookielib.LWPCookieJar object. What is the correct way of doing that? jar = my_function_being_tested() self.assertIsInstance(jar, cookielib.LWPCookieJar) That works in 2.7. If you're using something older than 2.7, you'll need to do: self.assertTrue(isinstance(jar, cookielib.LWPCookieJar) Alternatively, just download the 2.7 version of unittest and use that (it works fine with 2.6, not sure about earlier than that). 3) isinstance(return_value, cookielib.LWPCookieJar) seems to be the best way, however somewhere I read that using isinstance is discouraged Where did you read that, and in what context? Compared to type(), isinstance() is an improvement because it correctly handles subclasses. If you want a LWPCookieJar, you should be happy to have somebody give you a subclass of LWPCookieJar (assuming they correctly implemented the interface). Thus says the Church of Most Corpulent Staticness and Type Bondage. On the other hand, there are some (adherents of the Most Holy and Loquacious Church of Duck Typing) who would say that testing for class at all is a sin, and what you want to do is test that the object being tested has the methods and attributes you expect. Me, I'm somewhere in between. I believe that pinching it and seeing what the quack sounds like is usually the right thing to do. On the other hand, if you want to demand to see its Certificate of Duckiness, you have a right to do that too. -- http://mail.python.org/mailman/listinfo/python-list
Re: verify the return value of a function
On Fri, 20 Jan 2012 08:53:13 -0500, Mel Wilson wrote: Jean-Michel Pichavant wrote: isinstance is fine, if you could find the source where it is discouraged... Could be a consequence of some specific context. However, checking types in OOP is in general a failure. Unitary tests are possibly an exception. I think it's discouraged when people try to write big overloaded functions that check the types of the arguments to decide what they should be doing. I don't agree with that. Writing polymorphic functions using isinstance is a perfectly reasonable thing to do. E.g. from the standard library's decimal module: class Decimal(object): Floating point class for decimal arithmetic. # We're immutable, so use __new__ not __init__ def __new__(cls, value=0, context=None): self = object.__new__(cls) # From a string # REs insist on real strings, so we can too. if isinstance(value, str): ... # From an integer if isinstance(value, int): ... # From another decimal if isinstance(value, Decimal): ... # From an internal working value if isinstance(value, _WorkRep): ... # tuple/list conversion (possibly from as_tuple()) if isinstance(value, (list,tuple)): ... if isinstance(value, float): ... raise TypeError(Cannot convert %r to Decimal % value) What should be avoided, when possible, is over-reliance on isinstance checks instead of protocol or interface checks. For example, don't check for a list if your function doesn't *need* a list but would be happy with a tuple or some other sequence. Worse than isinstance is testing for an exact type: if type(x) is list # worse than isinstance(x, list) although of course, there are times where you need to break the rules. In diagnostics and tests like the OP's there should be no problem. Agreed. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: verify the return value of a function
On Fri, Jan 20, 2012 at 2:15 AM, Ulrich Eckhardt ulrich.eckha...@dominolaser.com wrote: Am 19.01.2012 21:45, schrieb Jabba Laci: In a unit test, I want to verify that a function returns a cookielib.LWPCookieJar object. What is the correct way of doing that? 1) First I tried to figure out its type with type(return_value) but it istype 'instance' I'm not sure where the problem here is and where exactly you are seeing this. This might even indicate a problem with how the returned type is constructed. This just means that LWPCookieJar is an old-style class: class Foo: pass ... type(Foo()) type 'instance' Foo().__class__ class __main__.Foo at 0x01DC4CE0 So for type checks here the __class__ attribute should be used, not the type function. isinstance is better for instance checks though in either case. Cheers, Ian -- http://mail.python.org/mailman/listinfo/python-list
Re: verify the return value of a function
On 1/20/2012 10:07 AM, Steven D'Aprano wrote: What should be avoided, when possible, is over-reliance on isinstance checks instead of protocol or interface checks. For example, don't check for a list if your function doesn't *need* a list but would be happy with a tuple or some other sequence. In other words, do not use isinstance to artificially limit the input domain of a function. The generic or polymorphic nature of (builtin) operators and functions is a major feature of Python. On the other hand, the output range of a function is typically much more limited as to type. Complete testing requires testing the specified output type. For instance, sorted(iterable) is documented as producing a sorted list, so 'type(output) is list' is an appropriate test. -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
verify the return value of a function
Hi, In a unit test, I want to verify that a function returns a cookielib.LWPCookieJar object. What is the correct way of doing that? 1) First I tried to figure out its type with type(return_value) but it is type 'instance' 2) return_value.__class__ .__name__ gives 'LWPCookieJar', which is bettter 3) isinstance(return_value, cookielib.LWPCookieJar) seems to be the best way, however somewhere I read that using isinstance is discouraged Thanks, Laszlo -- http://mail.python.org/mailman/listinfo/python-list
Re: verify the return value of a function
On Thu, Jan 19, 2012 at 12:45 PM, Jabba Laci jabba.l...@gmail.com wrote: Hi, In a unit test, I want to verify that a function returns a cookielib.LWPCookieJar object. What is the correct way of doing that? snip 3) isinstance(return_value, cookielib.LWPCookieJar) seems to be the best way, however somewhere I read that using isinstance is discouraged Explicit typechecking is often discouraged in favor of duck typing. However, if you want to do explicit typechecking (as one might in unit tests), then isinstance() is absolutely the technique to use. The alternative would be to check for the specific attributes of LWPCookieJar that you're relying upon (using hasattr() or similar), but that's probably overkill here. Cheers, Chris -- http://rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
