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

Walkthrough

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

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