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Python Program to Check if a String Is a Number

Beginner · Strings

Anything typed by a user arrives as a string, even when it looks like a number, so programs often need to check whether "42" or "3.14" can be turned into a number before doing maths with it. This program tries the conversion and catches the error when it fails.

Program

check_if_number_string.py
samples = ["2026", "-15", "3.14", "12abc", "1e3", " 42 ", ""]
# or: samples = [input("Enter something: ")]
for text in samples:
try:
float(text) # works for whole numbers, decimals and signs
result = "is a number"
except ValueError:
result = "is not a number"
print(f'"{text}"', result, "| isdigit():", text.isdigit())
Output
"2026" is a number | isdigit(): True
"-15" is a number | isdigit(): False
"3.14" is a number | isdigit(): False
"12abc" is not a number | isdigit(): False
"1e3" is a number | isdigit(): False
" 42 " is a number | isdigit(): False
"" is not a number | isdigit(): False

How it works

  1. float(text) converts the string to a number when it can, and raises a ValueError when it cannot.
  2. try and except ValueError catch that error, so a bad value gives a message instead of stopping the program.
  3. float() accepts signs, decimal points, spaces around the number and forms like "1e3" (1000), which makes it a better test than isdigit().
  4. isdigit() is True only when every character is a digit, so it says no to "-15" and "3.14"; use it only for whole numbers without a sign.
  5. float() also accepts the words "nan" and "inf", which stand for "not a number" and infinity, so reject those yourself if they matter.

Other ways to write it

Whole numbers only, with int()

Whole numbers only, with int()
def is_whole_number(text):
try:
int(text)
return True
except ValueError:
return False
for text in ["42", "-7", "3.5", "seven"]:
print(text, "->", "whole number" if is_whole_number(text) else "not a whole number")
Output
42 -> whole number
-7 -> whole number
3.5 -> not a whole number
seven -> not a whole number

int() accepts a sign but refuses a decimal point, so it is the right test when only whole numbers make sense, like an age or a roll number.

Checking by hand, without try

Checking by hand, without try
def looks_like_decimal(text):
if text and text[0] in "+-":
text = text[1:] # allow one sign at the start
if text.count(".") > 1:
return False
digits = text.replace(".", "")
return digits.isdigit()
for text in ["12", "-3.75", "4.2.1", ".5", "abc", "-"]:
print(text, "->", "number" if looks_like_decimal(text) else "not a number")
Output
12 -> number
-3.75 -> number
4.2.1 -> not a number
.5 -> number
abc -> not a number
- -> not a number

The function allows one sign and at most one decimal point, then insists that everything left is digits. It is a good exercise in string methods, but it does not accept forms like 1e3.

Try it yourself

Add "1,000" and "nan" to the list and explain the result for each.

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