How to properly use python's isinstance() to check if a variable is a number?

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You can use the types module:

>>> import types
>>> var = 1
>>> NumberTypes = (types.IntType, types.LongType, types.FloatType, types.ComplexType)
>>> isinstance(var, NumberTypes)
True

Note the use of a tuple to test against multiple types.

Under the hood, IntType is just an alias for int, etc.:

>>> isinstance(var, (int, long, float, complex))
True

The complex type requires that your python was compiled with support for complex numbers; if you want to guard for this use a try/except block:

>>> try:
...     NumberTypes = (types.IntType, types.LongType, types.FloatType, types.ComplexType)
... except AttributeError:
...     # No support for complex numbers compiled
...     NumberTypes = (types.IntType, types.LongType, types.FloatType)
...

or if you just use the types directly:

>>> try:
...     NumberTypes = (int, long, float, complex)
... except NameError:
...     # No support for complex numbers compiled
...     NumberTypes = (int, long, float)
...

Last but not least, you can use the numbers.Numbers abstract base type (new in Python 2.6) to also support custom numeric types that don't derive directly from the above types:

>>> import numbers
>>> isinstance(var, numbers.Number)
True

This module does make the assumption that the complex type is enabled; you'll get an import error if it is not.