[Tutor] Recognizing real numbers

[Steven D'Aprano]

On Tue, Feb 21, 2012 at 07:00:40PM -0800, Michael Lewis wrote:

> It seems that the .isdigit() function that I use doesn't recognize the .5
> as a number and therefore doesn't multiply it. How can I get my code to
> recognize numbers such as .5, 1.75 as numbers?

As the saying goes, it is often Easier to Ask for Forgiveness than 
Permission ("EAFP").

In Python terms, that means when you want to do something, just do it, 
and then catch the exception if it fails. Wrap this in a function, and 
it allows you to use a nifty Look Before You Leap ("LBYL") style of 
programming:

def isfloat(obj):
    """Return True if obj is a number or string-like floating point 
    number.
    """
    try:
        float(obj)
    except (TypeError, ValueError):
        return False
    else:  # no errors
        return True


And some examples of it in use:

>>> isfloat("123.456")
True
>>> isfloat("123.45...6")
False
>>> isfloat(".1")
True
>>> isfloat("1.1e6")  # 1.1 million
True
>>> isfloat("1.1g6")
False
>>> isfloat("+.1")
True
>>> isfloat(".")
False



-- 
Steven