Checking whether a variable is an integer or not


Question

How do I check whether a variable is an integer?

1
796
6/16/2012 7:48:56 PM

Accepted Answer

If you need to do this, do

isinstance(<var>, int)

unless you are in Python 2.x in which case you want

isinstance(<var>, (int, long))

Do not use type. It is almost never the right answer in Python, since it blocks all the flexibility of polymorphism. For instance, if you subclass int, your new class should register as an int, which type will not do:

class Spam(int): pass
x = Spam(0)
type(x) == int # False
isinstance(x, int) # True

This adheres to Python's strong polymorphism: you should allow any object that behaves like an int, instead of mandating that it be one.

BUT

The classical Python mentality, though, is that it's easier to ask forgiveness than permission. In other words, don't check whether x is an integer; assume that it is and catch the exception results if it isn't:

try:
    x += 1
except TypeError:
    ...

This mentality is slowly being overtaken by the use of abstract base classes, which let you register exactly what properties your object should have (adding? multiplying? doubling?) by making it inherit from a specially-constructed class. That would be the best solution, since it will permit exactly those objects with the necessary and sufficient attributes, but you will have to read the docs on how to use it.

952
11/12/2017 11:58:47 PM

All proposed answers so far seem to miss the fact that a double (floats in python are actually doubles) can also be an integer (if it has nothing after the decimal point). I use the built-in is_integer() method on doubles to check this.

Example (to do something every xth time in a for loop):

for index in range(y): 
    # do something
    if (index/x.).is_integer():
        # do something special

Edit:

You can always convert to a float before calling this method. The three possibilities:

>>> float(5).is_integer()
True
>>> float(5.1).is_integer()
False
>>> float(5.0).is_integer()
True

Otherwise, you could check if it is an int first like Agostino said:

def is_int(val):
    if type(val) == int:
        return True
    else:
        if val.is_integer():
            return True
        else:
            return False

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