How does Python determine that the input is all letters?

Strict analysis: any symbols other than numbers or letters (spaces, semicolons, etc.) will be False
isalnum() must be a mixture of numbers and letters
isalpha() is not case-sensitive

str_1 = "123" str_2 = "Abc" str_3 = "123Abc" #用isdigit函数判断是否数字 print(str_1.isdigit()) Ture print(str_2.isdigit()) False print(str_3.isdigit()) False #用isalpha判断是否字母 print(str_1.isalpha()) False print(str_2.isalpha()) Ture print(str_3.isalpha()) False #isalnum判断是否数字和字母的组合 print(str_1.isalnum()) Ture print(str_2.isalnum()) Ture print(str_1.isalnum()) Ture

Note: If the string contains characters other than letters or numbers, such as spaces, it will also return False

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