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

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

Validating userid

In any signup or login form , the userid input is to be checked then isalumn() function can be used.
Here is a code which checks the length and if the input string and is alphanumeric or not. If any condition fails then the flag used for checking became False.

if(len(userid) <5 or len(userid)>12 or not userid.isalnum()):
            flag_validation=False

Read the full script at Login Registration system.
All String methods

isalpha() isdecimal() Python- Tutorials

isalnum()

Validating userid

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