Check if a string is numeric, alphabetic, alphanumeric, or ASCII
Python provides various methods to check if the characters in the string (str) are numeric, alphabetic, alphanumeric, or ASCII.
- Check if a string is decimal:
str.isdecimal() - Check if a string is digits:
str.isdigit() - Check if a string is numeric:
str.isnumeric() - Check if a string is alphabetic:
str.isalpha() - Check if a string is alphanumeric:
str.isalnum() - Check if a string is ASCII:
str.isascii() - Check if a string is empty
- Check if a string is a number (= can be converted to numeric value)
For information on converting a numeric string (str) into a numeric value (int or float), or determining the case (upper or lower) of letters, refer to the following articles.
- Convert a string to a number (int, float) in Python
- Convert and determine uppercase and lowercase strings in Python
Check if a string is decimal: str.isdecimal()
The isdecimal() method returns True if all characters in the string are decimal characters that fall under the Nd category in Unicode. Fullwidth numbers in CJK languages are also considered True.
s = '1234567890'
print('s =', s)
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = 1234567890
# isdecimal: True
# isdigit: True
# isnumeric: True
s = '１２３４５６７８９０'
print('s =', s)
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = １２３４５６７８９０
# isdecimal: True
# isdigit: True
# isnumeric: True
For isdecimal(), strings that include symbols such as - and . are evaluated as False.
s = '-1.23'
print('s =', s)
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = -1.23
# isdecimal: False
# isdigit: False
# isnumeric: False
To recognize a numeric string like '-1.23' as valid, you can implement exception handling as detailed in the final section.
Check if a string is digits: str.isdigit()
The isdigit() method returns True for characters that are True with isdecimal() and for characters with the Unicode property value Numeric_Type set to Digit or Decimal.
For example, the superscript number ² ('\u00B2') is evaluated as False in isdecimal(), but True in isdigit().
s = '10\u00B2'
print('s =', s)
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = 10²
# isdecimal: False
# isdigit: True
# isnumeric: True
Check if a string is numeric: str.isnumeric()
The isnumeric() method returns True for characters that are True with isdigit() and for characters with the Unicode property value Numeric_Type set to Numeric.
Vulgar fractions, Roman numerals, Chinese numerals, and others are also determined as True under isnumeric().
s = '\u00BD'
print('s =', s)
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = ½
# isdecimal: False
# isdigit: False
# isnumeric: True
s = '\u2166'
print('s =', s)
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = Ⅶ
# isdecimal: False
# isdigit: False
# isnumeric: True
s = '一二三四五六七八九〇'
print('s =', s)
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = 一二三四五六七八九〇
# isdecimal: False
# isdigit: False
# isnumeric: True
Check if a string is alphabetic: str.isalpha()
The isalpha() method determines if all characters in the string belong to the alphabetic category, i.e., those with a Unicode character database general category property of Lm, Lt, Lu, Ll, or Lo.
This not only includes the Latin alphabet but also characters from other languages, such as Japanese hiragana, all of which are considered True.
s = 'abc'
print('s =', s)
print('isalpha:', s.isalpha())
# s = abc
# isalpha: True
s = 'あいうえお'
print('s =', s)
print('isalpha:', s.isalpha())
# s = あいうえお
# isalpha: True
Check if a string is alphanumeric: str.isalnum()
The isalnum() method returns True if all characters pass any of the previously mentioned methods, namely isdecimal(), isdigit(), isnumeric(), or isalpha().
Since each character is evaluated individually, a string with a mix of alphabetic and numeric characters will be evaluated as True in isalnum() even if it is False in all other methods.
s = 'abc123'
print('s =', s)
print('isalnum:', s.isalnum())
print('isalpha:', s.isalpha())
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = abc123
# isalnum: True
# isalpha: False
# isdecimal: False
# isdigit: False
# isnumeric: False
Check if a string is ASCII: str.isascii()
The isascii() method returns True if all characters in the string are ASCII characters (U+0000 - U+007F).
Symbols such as + and - are also determined as True under isascii().
s = 'abc123+-,.&'
print('s =', s)
print('isascii:', s.isascii())
print('isalnum:', s.isalnum())
# s = abc123+-,.&
# isascii: True
# isalnum: False
Non-ASCII characters are determined as False.
s = 'あいうえお'
print('s =', s)
print('isascii:', s.isascii())
print('isalnum:', s.isalnum())
# s = あいうえお
# isascii: False
# isalnum: True
Unlike other methods, isascii() returns True even for empty strings, as explained in the next section.
Check if a string is empty
The empty string '' is considered True by isascii(), while considered False by other methods.
s = ''
print('s =', s)
print('isalnum:', s.isalnum())
print('isalpha:', s.isalpha())
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
print('isascii:', s.isascii())
# s =
# isalnum: False
# isalpha: False
# isdecimal: False
# isdigit: False
# isnumeric: False
# isascii: True
Use bool() to check if a string is empty. It returns False for an empty string and True for non-empty strings.
print(bool(''))
# False
print(bool('abc123'))
# True
Check if a string is a number (= can be converted to numeric value)
Negative numbers or decimal values contain symbols like . or -. Hence, they are evaluated as False by all methods, except isascii().
Although isascii() returns True, it is not suitable for verifying if a string is a number (i.e., can be converted to a numeric value) because it also returns True even when other symbols or letters are included.
s = '-1.23'
print('s =', s)
print('isalnum:', s.isalnum())
print('isalpha:', s.isalpha())
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
print('isascii:', s.isascii())
# s = -1.23
# isalnum: False
# isalpha: False
# isdecimal: False
# isdigit: False
# isnumeric: False
# isascii: True
You can convert a string into a floating-point number using float(). However, keep in mind that this will result in an error if you attempt to convert non-numeric strings, such as 'abc'.
print(float('-1.23'))
# -1.23
print(type(float('-1.23')))
# <class 'float'>
# print(float('abc'))
# ValueError: could not convert string to float: 'abc'
By implementing exception handling, you can define a function that returns True if a string can successfully be converted using float().
def is_num(s):
try:
float(s)
except ValueError:
return False
else:
return True
print(is_num('123'))
# True
print(is_num('-1.23'))
# True
print(is_num('+1.23e10'))
# True
print(is_num('abc'))
# False
print(is_num('10,000,000'))
# False
If you need to treat a string with separators (,) as numeric, use the replace() method to remove separators by replacing , with an empty string ''.
def is_num_delimiter(s):
try:
float(s.replace(',', ''))
except ValueError:
return False
else:
return True
print(is_num_delimiter('10,000,000'))
# True
You can also use replace() for whitespace separators.
def is_num_delimiter2(s):
try:
float(s.replace(',', '').replace(' ', ''))
except ValueError:
return False
else:
return True
print(is_num_delimiter2('10,000,000'))
# True
print(is_num_delimiter2('10 000 000'))
# True
If you want to check whether a number is an integer or a decimal, see the following article.