isDigit() for rational numbers?

Question

I am trying to evaluate if the string in one of the textbox of my interface is a number (i.e. not text or anything else). In Python, there is a method called isdigit() that will return True if the string only contains digits (no negative signs or decimal points). Is there another way I could evaluate if my string is a rational number (ex: 1.25).

Example code:

if self.components.txtZoomPos.text.isdigit():
        step = int(self.components.txtZoomPos.text)

Answer 1

int() or float() throws ValueError if the literal is not valid

Answer 2

float() throws a ValueError if the conversion fails. So try to convert your string to a float, and catch the ValueError.

Answer 3

1.25 is a notation commonly used for reals, less so for rational numbers. Python's float will raise a ValueError when conversion fails. Thus:

def isReal(txt):
    try:
        float(txt)
        return True
    except ValueError:
        return False

Answer 4

try/catch is very cheap in Python, and attempting to construct a float from a string that's not a number raises an exception:

>>> float('1.45')
1.45
>>> float('foo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for float(): foo

You can just do something like:

try:
    # validate it's a float
    value = float(self.components.txtZoomPos.text)
except ValueError, ve:
    pass # your error handling goes here