python - A version of str.isdigit that returns True for decimal fractions?

I want to test raw_input to make sure that the string contains only numbers and at maximum a single decimal point. str.isdigit() looked promising but it will not return True if there is a decimal point in the string.

Ideally, the code would look like this:

def enter_number():
    number = raw_input("Enter a number: ")  # I enter 3.5
    if number.SOMETHING:  # SOMETHING is what I am looking for
        float_1 = float(number)
        return float_1
    else
        sys.exit()

half = enter_number() / 2  # = 1.75
double = enter_number() * 2  # = 7

Answer

I suggest the EAFP approach.

float raises the ValueError exception if its argument is not a valid float:

In [6]: float('bad value')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
 in ()

----> 1 float('bad value')

ValueError: could not convert string to float: 'bad value'

You could catch it:

number = raw_input("Enter a number: ")
try:
    return float(number)

except ValueError:
    sys.exit()