How to Check If a String Represents an Int, Without Using Try/Except

def RepresentsInt(s):
    try: 
        int(s)
        return True
    except ValueError:
        return False

>>> print RepresentsInt("+123")
True
>>> print RepresentsInt("10.0")
False

if not rate or not str(rate).replace(".", "").isnumeric():
    rate = 0
elif isinstance(rate, str):
    if rate.isnumeric():
        rate = int(rate)
    elif rate.replace(".", "").isnumeric():
        rate = float(rate)

rate	output	type
'test'	0	<class 'int'>
'test123'	0	<class 'int'>
'123'	123	<class 'int'>
None	0	<class 'int'>
False	0	<class 'int'>
123	123	<class 'int'>
'1.1/1'	0	<class 'int'>
complex(1, 1)	0	<class 'int'>
'1.1'	1.1	<class 'float'>
1.1	1.1	<class 'float'>

Checking whether a variable is an integer or not

If you need to do this, do

isinstance(<var>, int)

unless you are in Python 2.x in which case you want

isinstance(<var>, (int, long))

Do not use type. It is almost never the right answer in Python, since it blocks all the flexibility of polymorphism. For instance, if you subclass int, your new class should register as an int, which type will not do:

class Spam(int): pass
x = Spam(0)
type(x) == int # False
isinstance(x, int) # True

This adheres to Python's strong polymorphism: you should allow any object that behaves like an int, instead of mandating that it be one.

BUT

The classical Python mentality, though, is that it's easier to ask forgiveness than permission. In other words, don't check whether x is an integer; assume that it is and catch the exception results if it isn't:

try:
    x += 1
except TypeError:
    ...

This mentality is slowly being overtaken by the use of abstract base classes, which let you register exactly what properties your object should have (adding? multiplying? doubling?) by making it inherit from a specially-constructed class. That would be the best solution, since it will permit exactly those objects with the necessary and sufficient attributes, but you will have to read the docs on how to use it.

How to check if a variable is an integer or a string?

In my opinion you have two options:

• Just try to convert it to an int, but catch the exception:

  try:
      value = int(value)
  except ValueError:
      pass  # it was a string, not an int.
  

  This is the Ask Forgiveness approach.

• Explicitly test if there are only digits in the string:

  value.isdigit()
  

  str.isdigit() returns True only if all characters in the string are digits (0-9).

  The unicode / Python 3 str type equivalent is unicode.isdecimal() / str.isdecimal(); only Unicode decimals can be converted to integers, as not all digits have an actual integer value (U+00B2 SUPERSCRIPT 2 is a digit, but not a decimal, for example).

  This is often called the Ask Permission approach, or Look Before You Leap.

The latter will not detect all valid int() values, as whitespace and + and - are also allowed in int() values. The first form will happily accept ' +10 ' as a number, the latter won't.

If your expect that the user normally will input an integer, use the first form. It is easier (and faster) to ask for forgiveness rather than for permission in that case.

Checking to see if a string is an integer or float

If the string is convertable to integer, it should be digits only. It should be noted that this approach, as @cwallenpoole said, does NOT work with negative inputs beacuse of the '-' character. You could do:

if NumberString.isdigit():
    Number = int(NumberString)
else:
    Number = float(NumberString)

If you already have Number confirmed as a float, you can always use is_integer (works with negatives):

if Number.is_integer():
    Number = int(Number)

How do I check if a string is a number (float)?

Which, not only is ugly and slow

I'd dispute both.

A regex or other string parsing method would be uglier and slower.

I'm not sure that anything much could be faster than the above. It calls the function and returns. Try/Catch doesn't introduce much overhead because the most common exception is caught without an extensive search of stack frames.

The issue is that any numeric conversion function has two kinds of results

• A number, if the number is valid
• A status code (e.g., via errno) or exception to show that no valid number could be parsed.

C (as an example) hacks around this a number of ways. Python lays it out clearly and explicitly.

I think your code for doing this is perfect.

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