Tutorial :How can I type-check variables in Python?


I have a Python function that takes a numeric argument that must be an integer in order for it behave correctly. What is the preferred way of verifying this in Python?

My first reaction is to do something like this:

def isInteger(n):      return int(n) == n  

But I can't help thinking that this is 1) expensive 2) ugly and 3) subject to the tender mercies of machine epsilon.

Does Python provide any native means of type checking variables? Or is this considered to be a violation of the language's dynamically typed design?

EDIT: since a number of people have asked - the application in question works with IPv4 prefixes, sourcing data from flat text files. If any input is parsed into a float, that record should be viewed as malformed and ignored.


isinstance(n, int)  

If you need to know whether it's definitely an actual int and not a subclass of int (generally you shouldn't need to do this):

type(n) is int  


return int(n) == n  

isn't such a good idea, as cross-type comparisons can be true - notably int(3.0)==3.0


Yeah, as Evan said, don't type check. Just try to use the value:

def myintfunction(value):     """ Please pass an integer """     return 2 + value  

That doesn't have a typecheck. It is much better! Let's see what happens when I try it:

>>> myintfunction(5)  7  

That works, because it is an integer. Hm. Lets try some text.

>>> myintfunction('text')  Traceback (most recent call last):    File "<stdin>", line 1, in <module>    File "<stdin>", line 3, in myintfunction  TypeError: unsupported operand type(s) for +: 'int' and 'str'  

It shows an error, TypeError, which is what it should do anyway. If caller wants to catch that, it is possible.

What would you do if you did a typecheck? Show an error right? So you don't have to typecheck because the error is already showing up automatically.

Plus since you didn't typecheck, you have your function working with other types:


>>> print myintfunction(2.2)  4.2  

Complex numbers:

>>> print myintfunction(5j)  (2+5j)  


>>> import decimal  >>> myintfunction(decimal.Decimal('15'))  Decimal("17")  

Even completely arbitrary objects that can add numbers!

>>> class MyAdderClass(object):  ...     def __radd__(self, value):  ...             print 'got some value: ', value  ...             return 25  ...   >>> m = MyAdderClass()  >>> print myintfunction(m)  got some value:  2  25  

So you clearly get nothing by typechecking. And lose a lot.


Since you've edited the question, it is now clear that your application calls some upstream routine that makes sense only with ints.

That being the case, I still think you should pass the parameter as received to the upstream function. The upstream function will deal with it correctly e.g. raising an error if it needs to. I highly doubt that your function that deals with IPs will behave strangely if you pass it a float. If you can give us the name of the library we can check that for you.

But... If the upstream function will behave incorrectly and kill some kids if you pass it a float (I still highly doubt it), then just just call int() on it:

def myintfunction(value):     """ Please pass an integer """     return upstreamfunction(int(value))  

You're still not typechecking, so you get most benefits of not typechecking.

If even after all that, you really want to type check, despite it reducing your application's readability and performance for absolutely no benefit, use an assert to do it.

assert isinstance(...)  assert type() is xxxx  

That way we can turn off asserts and remove this <sarcasm>feature</sarcasm> from the program by calling it as

python -OO program.py  


if type(n) is int  

This checks if n is a Python int, and only an int. It won't accept subclasses of int.

Type-checking, however, does not fit the "Python way". You better use n as an int, and if it throws an exception, catch it and act upon it.


Python now supports gradual typing via the typing module and mypy. The typing module is a part of the stdlib as of Python 3.5 and can be downloaded from PyPi if you need backports for Python 2 or previous version of Python 3. You can install mypy by running pip install mypy from the command line.

In short, if you want to verify that some function takes in an int, a float, and returns a string, you would annotate your function like so:

def foo(param1: int, param2: float) -> str:      return "testing {0} {1}".format(param1, param2)  

If your file was named test.py, you could then typecheck once you've installed mypy by running mypy test.py from the command line.

If you're using an older version of Python without support for function annotations, you can use type comments to accomplish the same effect:

def foo(param1, param2):      # type: (int, float) -> str      return "testing {0} {1}".format(param1, param2)  

You use the same command mypy test.py for Python 3 files, and mypy --py2 test.py for Python 2 files.

The type annotations are ignored entirely by the Python interpreter at runtime, so they impose minimal to no overhead -- the usual workflow is to work on your code and run mypy periodically to catch mistakes and errors. Some IDEs, such as PyCharm, will understand type hints and can alert you to problems and type mismatches in your code while you're directly editing.

If, for some reason, you need the types to be checked at runtime (perhaps you need to validate a lot of input?), you should follow the advice listed in the other answers -- e.g. use isinstance, issubclass, and the like. There are also some libraries such as enforce that attempt to perform typechecking (respecting your type annotations) at runtime, though I'm uncertain how production-ready they are as of time of writing.

For more information and details, see the mypy website, the mypy FAQ, and PEP 484.


Don't type check. The whole point of duck typing is that you shouldn't have to. For instance, what if someone did something like this:

class MyInt(int):      # ... extra stuff ...  


Programming in Python and performing typechecking as you might in other languages does seem like choosing a screwdriver to bang a nail in with. It is more elegant to use Python's exception handling features.

From an interactive command line, you can run a statement like:


That will generate an error - ipython tells me:

<type 'exceptions.ValueError'>: invalid literal for int() with base 10: 'sometext'  

Now you can write some code like:

try:     int(myvar) + 50  except ValueError:     print "Not a number"  

That can be customised to perform whatever operations are required AND to catch any errors that are expected. It looks a bit convoluted but fits the syntax and idioms of Python and results in very readable code (once you become used to speaking Python).


I would be tempted to to something like:

def check_and_convert(x):      x = int(x)      assert 0 <= x <= 255, "must be between 0 and 255 (inclusive)"      return x    class IPv4(object):      """IPv4 CIDR prefixes is A.B.C.D/E where A-D are          integers in the range 0-255, and E is an int          in the range 0-32."""        def __init__(self, a, b, c, d, e=0):          self.a = check_and_convert(a)          self.b = check_and_convert(a)          self.c = check_and_convert(a)          self.d = check_and_convert(a)          assert 0 <= x <= 32, "must be between 0 and 32 (inclusive)"          self.e = int(e)  

That way when you are using it anything can be passed in yet you only store a valid integer.


how about:

def ip(string):      subs = string.split('.')      if len(subs) != 4:          raise ValueError("incorrect input")      out = tuple(int(v) for v in subs if 0 <= int(v) <= 255)      if len(out) != 4:          raise ValueError("incorrect input")      return out  

ofcourse there is the standard isinstance(3, int) function ...

Note:If u also have question or solution just comment us below or mail us on toontricks1994@gmail.com
Next Post »