Question

What is the best way to validate an input as a currency (without any currency symbols)
My valid inputs can be only in the form of 40 or 40.12

Answer 1

How about simply using format() and try/except for wrong values?

>>> "{:.2f}".format(float("40.12"))
'40.12'
>>> "{:.2f}".format(float("40"))
'40.00'
>>> "{:.2f}".format(float("40.123"))
'40.12'
>>> try:
...     "{:.2f}".format(float("40.123€"))
... except ValueError:
...     "fail"
... 
'fail'

Note that it simply cuts any number behind the second number behind the decimal point.
You should be more specific by showing us more examples with your desired behaviour.