python 40 lines · 8 steps

Normalizing signup emails in Python

A single function cleans, validates, and canonicalizes email addresses while rejecting blanks and disposable domains.

Explained by highlit

Intermediate validation normalization data-cleaning custom-exceptions

1import re
2from email_validator import validate_email, EmailNotValidError
3 
4_DISPOSABLE_DOMAINS = {
5    "mailinator.com",
6    "guerrillamail.com",
7    "10minutemail.com",
8    "tempmail.com",
9}
10 
11_WHITESPACE = re.compile(r"\s+")
12 
13 
14class EmailValidationError(Exception):
15    pass
16 
17 
18def normalize_signup_email(raw: str, *, allow_disposable: bool = False) -> str:
19    if not raw or not raw.strip():
20        raise EmailValidationError("Email is required.")
21 
22    candidate = _WHITESPACE.sub("", raw).strip("<>").lower()
23 
24    try:
25        result = validate_email(candidate, check_deliverability=True)
26    except EmailNotValidError as exc:
27        raise EmailValidationError(str(exc)) from exc
28 
29    normalized = result.normalized
30    domain = result.domain.lower()
31 
32    if not allow_disposable and domain in _DISPOSABLE_DOMAINS:
33        raise EmailValidationError("Disposable email addresses are not allowed.")
34 
35    local, _, host = normalized.partition("@")
36    if host in {"gmail.com", "googlemail.com"}:
37        local = local.split("+", 1)[0].replace(".", "")
38        normalized = f"{local}@gmail.com"
39 
40    return normalized

01 / 01

STEP 01

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Three takeaways

1Wrapping a library's exception in your own domain error keeps callers decoupled from third-party details.
2Canonicalizing provider-specific quirks like Gmail dots and plus-tags prevents duplicate accounts for the same person.
3Normalizing input before validating catches sloppy but salvageable data instead of rejecting it outright.

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