"""

Implements an is valid email address algorithm

@ https://en.wikipedia.org/wiki/Email_address

"""

import string

email_tests: tuple[tuple[str, bool], ...] = (

("[email protected]", True),

("[email protected]", True),

("[email protected]", True),

("[email protected]", True),

("[email protected]", True),

("[email protected]", True),

("[email protected]", True),

("[email protected]", True),

("test/[email protected]", True),

(

"123456789012345678901234567890123456789012345678901234567890123@example.com",

True,

),

("admin@mailserver1", True),

("[email protected]", True),

("Abc.example.com", False),

("A@b@[email protected]", False),

("[email protected]", False),

("a(c)d,e:f;g<h>i[j\\k][email protected]", False),

(

"12345678901234567890123456789012345678901234567890123456789012345@example.com",

False,

),

("i.like.underscores@but_its_not_allowed_in_this_part", False),

("", False),

)

# The maximum octets (one character as a standard unicode character is one byte)

# that the local part and the domain part can have

MAX_LOCAL_PART_OCTETS = 64

MAX_DOMAIN_OCTETS = 255

def is_valid_email_address(email: str) -> bool:

"""

Returns True if the passed email address is valid.

The local part of the email precedes the singular @ symbol and

is associated with a display-name. For example, "john.smith"

The domain is stricter than the local part and follows the @ symbol.

Global email checks:

1. There can only be one @ symbol in the email address. Technically if the

@ symbol is quoted in the local-part, then it is valid, however this

implementation ignores "" for now.

(See https://en.wikipedia.org/wiki/Email_address#:~:text=If%20quoted,)

2. The local-part and the domain are limited to a certain number of octets. With

unicode storing a single character in one byte, each octet is equivalent to

a character. Hence, we can just check the length of the string.

Checks for the local-part:

3. The local-part may contain: upper and lowercase latin letters, digits 0 to 9,

and printable characters (!#$%&'*+-/=?^_`{|}~)

4. The local-part may also contain a "." in any place that is not the first or

last character, and may not have more than one "." consecutively.

Checks for the domain:

5. The domain may contain: upper and lowercase latin letters and digits 0 to 9

6. Hyphen "-", provided that it is not the first or last character

7. The domain may also contain a "." in any place that is not the first or

last character, and may not have more than one "." consecutively.

>>> for email, valid in email_tests:

... assert is_valid_email_address(email) == valid

"""

# (1.) Make sure that there is only one @ symbol in the email address

if email.count("@") != 1:

return False

local_part, domain = email.split("@")

# (2.) Check octet length of the local part and domain

if len(local_part) > MAX_LOCAL_PART_OCTETS or len(domain) > MAX_DOMAIN_OCTETS:

return False

# (3.) Validate the characters in the local-part

if any(

char not in string.ascii_letters + string.digits + ".(!#$%&'*+-/=?^_`{|}~)"

for char in local_part

):

return False

# (4.) Validate the placement of "." characters in the local-part

if local_part.startswith(".") or local_part.endswith(".") or ".." in local_part:

return False

# (5.) Validate the characters in the domain

if any(char not in string.ascii_letters + string.digits + ".-" for char in domain):

return False

# (6.) Validate the placement of "-" characters

if domain.startswith("-") or domain.endswith("."):

return False

# (7.) Validate the placement of "." characters

return not (domain.startswith(".") or domain.endswith(".") or ".." in domain)

if __name__ == "__main__":

import doctest

doctest.testmod()

for email, valid in email_tests:

is_valid = is_valid_email_address(email)

assert is_valid == valid, f"{email} is {is_valid}"

print(f"Email address {email} is {'not ' if not is_valid else ''}valid")