Check if a string contains only digits in Python

isdigit() is True when a string has at least one character and every character is a digit. It is a quick way to check typed input before you turn it into a number.

The program

for text in ["12345", "123a5", "", " 42", "-7", "3.5"]:
    print(f"{text!r:9} {text.isdigit()}")

'12345'   True
'123a5'   False
''        False
' 42'     False
'-7'      False
'3.5'     False

How it works

• {text!r} prints the string with its quotes, so you can see the empty string and the space in ' 42'. :9 pads it to 9 characters, so the second column lines up.
• An empty string gives False, because it has no digits at all.
• A space, a minus sign and a decimal point are not digits. So isdigit() is False for " 42", "-7" and "3.5", even though people read them as numbers.

isdigit() or isdecimal()?

isdigit() also accepts some special digit characters, such as the small raised ² in "x²". int() cannot convert those. isdecimal() accepts only the characters that int() can use:

print("²".isdigit())
print("²".isdecimal())

True
False

A safer check: try int()

To accept everything int() accepts (spaces around the number, a minus sign), try the conversion and catch the ValueError:

def is_whole_number(text):
    try:
        int(text)
        return True
    except ValueError:
        return False


for text in ["42", " 42 ", "-7", "3.5", "forty"]:
    print(f"{text!r:8} {is_whole_number(text)}")

'42'     True
' 42 '   True
'-7'     True
'3.5'    False
'forty'  False

int() ignores spaces at the start and the end, and accepts a sign. It still rejects "3.5": use float() in the same way if decimals are allowed.