Python determines and checks whether a string is numeric or alphabetic

Money and Business

Python provides several string methods to determine and check whether a string type is numeric or alphabetic.

Each method is explained with sample code.

  • Determines whether a string is a decimal digit:str.isdecimal()
  • Determining if a string is a number:str.isdigit()
  • Determines whether a string is a character representing a number:str.isnumeric()
  • Determines if string is alphabetic:str.isalpha()
  • Determine if string is alphanumeric:str.isalnum()
  • Determines whether strings are ASCII characters:str.isascii()
  • Judgment of empty string
  • Determine if strings can be converted to numbers

For methods other than isascii(), a string containing an empty string, the following symbols, etc., is false.

  • ,
  • .
  • -

-1.23, etc., as a numerical value is explained at the end of this section.

Regular expressions can be used to determine character types more flexibly and to extract the relevant character types.

See the following article for more information on how to determine the following

  • How to convert a numeric string (str) to a number (int, float)
  • How to determine upper and lower case

Determines whether a string is a decimal digit: str.isdecimal()

In isdecimal(), it is true if all characters are decimal digits, that is, characters in the general category Nd of Unicode. It is also true for full-width Arabic numerals, etc.

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

If it contains a symbol such as a minus sign or a period, it is false. For example, if you want to determine that a string such as '-1.23' is a numeric value, you can use exception handling. This is explained at the end of this section.

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

Determining if a string is a number: str.isdigit()

In isdigit(), in addition to numbers that are true in isdecimal(), numbers whose Unicode property value Numeric_Type is Digit or Decimal are also true.

For example, a superscript number representing a square is false in isdecimal() but true in isdigit().

  • superscript number representing the square
    • ²
    • '\u00B2}'
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

Determines whether a string is a character representing a number: str.isnumeric()

In isnumeric(), in addition to numbers that are true in isdigit(), numbers whose Unicode property value Numeric_Type is Numeric are also true.

Fractions, Roman numerals, and Chinese numerals are also true.

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

s = '壱億参阡萬'
print('s =', s)
print('isdecimal:', s.isdecimal())
print('isdigit:', s.isdigit())
print('isnumeric:', s.isnumeric())
# s = 壱億参阡萬
# isdecimal: False
# isdigit: False
# isnumeric: True

Determines if string is alphabetic: str.isalpha()

In isalpha(), a Unicode general category property with one of the following is true.

  • Lm
  • Lt
  • Lu
  • Ll
  • Lo

The alphabet, Chinese characters, etc. will be true.

s = 'abc'
print('s =', s)
print('isalpha:', s.isalpha())
# s = abc
# isalpha: True

s = '漢字'
print('s =', s)
print('isalpha:', s.isalpha())
# s = 漢字
# isalpha: True

Arabic numerals are false, but Chinese numerals are true because they are also Chinese characters; however, zeros in Chinese numerals are false.

s = '1234567890'
print('s =', s)
print('isalpha:', s.isalpha())
# s = 1234567890
# isalpha: False

s = '１２３４５６７８９０'
print('s =', s)
print('isalpha:', s.isalpha())
# s = １２３４５６７８９０
# isalpha: False

s = '一二三四五六七八九'
print('s =', s)
print('isalpha:', s.isalpha())
# s = 一二三四五六七八九
# isalpha: True

s = '壱億参阡萬'
print('s =', s)
print('isalpha:', s.isalpha())
# s = 壱億参阡萬
# isalpha: True

s = '〇'
print('s =', s)
print('isalpha:', s.isalpha())
# s = 〇
# isalpha: False

Roman numerals are false.

s = '\u2166'
print('s =', s)
print('isalpha:', s.isalpha())
# s = Ⅶ
# isalpha: False

Determine if string is alphanumeric: str.isalnum()

In isalnum(), it is true if each character is true in any of the following methods listed so far.

  • isdecimal()
  • isdigit()
  • isnumeric()
  • isalpha()

Each character is evaluated individually, so a string containing letters and numbers will be true in isalnum() even if 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

Determines whether strings are ASCII characters: str.isascii()

Python 3.7 added isascii(). It returns true if all characters in the string are ASCII characters.

In addition to numbers and letters, symbols such as + and – are also true.

s = 'abc123+-,.&'
print('s =', s)
print('isascii:', s.isascii())
print('isalnum:', s.isalnum())
# s = abc123+-,.&
# isascii: True
# isalnum: False

Non-ASCII hiragana and other characters are 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

As we will see next, unlike the other methods, isascii() returns true even for an empty string.

Judgment of empty string

An empty string is true for isascii() and false for the 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 determine if it is an empty string. The return value is false for an empty string and true otherwise.

print(bool(''))
# False

print(bool('abc123'))
# True

Determine if strings can be converted to numbers

Negative or fractional value strings contain periods or minus signs. Therefore, the result is false for all methods except isascii().

Although true for isascii(), it is not suitable for determining whether a string can be converted to a numeric value, since it is true even if it contains other symbols or alphabetic characters.

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

Strings can be converted to floating point numbers with float(). Error for strings that cannot be converted.

print(float('-1.23'))
# -1.23

print(type(float('-1.23')))
# <class 'float'>

# print(float('abc'))
# ValueError: could not convert string to float: 'abc'

With exception handling, a function can be defined that returns true when a string can be converted with 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 want to determine that a comma-separated number is also true, use replace() to remove the comma (replace it 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

If you want to support whitespace delimitation, you can use replace() further.

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
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