isalnum() : Check if numbers or alphabets are only present in a string in Python

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my_str.isalnum())

Checks for alphanumeric characters in a string. Returns True if only number and alphabets are present and returns False if any thing other than alphanumeric characters are present.

my_str='plus2net'
print(my_str.isalnum())      # Output is True

my_str='Welcome to plus2net'
print(my_str.isalnum())      # Output is False (presence of Space)

More using special chars

print('plus2net'.isalnum())   # True  2 is allowed  
print('_plus2net'.isalnum())  #False  _ not allowed
print('plus_2net'.isalnum())  #False  _ not allowed
print('plus.2net'.isalnum())  #False  . not allowed
print('plus-2net'.isalnum())  #False  - not allowed
print('plus@2net'.isalnum())  #False  @ not allowed
print('plus#2net'.isalnum())  #False  # not allowed
print('plus 2net'.isalnum())  #False  space not allowed

All String methods

isalpha() isdecimal() Python- Tutorials