ISSN Validation in Python — Format, Check Digit & MOD-11

1. What is an ISSN?

ISSN stands for International Standard Serial Number. It is an eight-digit identifier assigned to serial publications — journals, magazines, newspapers, annual reports, monographic series, and any publication issued in successive parts.

The ISSN system is managed by the ISSN International Centre in Paris and coordinated through a network of national centres. Unlike an ISBN, which identifies a specific book edition, an ISSN identifies the title of a serial as a whole, regardless of how many issues have been published.

ISSNs are essential for interlibrary loan systems, union catalogues, citation databases (such as Web of Science and Scopus), and digital preservation workflows. They also appear in EAN-13 barcodes on printed periodicals, using the GS1 prefix 977.

2. ISSN structure

An ISSN consists of eight digits displayed in two groups of four, separated by a hyphen. The last digit is a check digit that may be a numeral (0-9) or the letter X (representing 10).

3. The MOD-11 check digit algorithm

The ISSN check digit uses the same MOD-11 weighted algorithm as ISBN-10. The first seven digits are multiplied by descending weights (8 down to 2), summed, and the check digit is chosen so the total is divisible by 11.

Let's walk through 0378-5955:

# issn_checksum.py — validate an ISSN check digit
import re


def validate_issn(raw: str) -> bool:
    digits = re.sub(r"[-\s]", "", raw)
    if not re.match(r"^\d{7}[\dXx]$", digits):
        return False

    total = 0
    for i in range(7):
        total += int(digits[i]) * (8 - i)

    last = digits[7].upper()
    check = 10 if last == "X" else int(last)
    total += check

    return total % 11 == 0


print(validate_issn("0378-5955"))  # True   valid ISSN
print(validate_issn("0317-847X"))  # True   X check digit
print(validate_issn("0378-5950"))  # False  bad check digit

4. Why regex isn't enough

A regex like \d{4}-\d{3}[\dXx] can verify that a string looks like an ISSN, but it cannot catch the most common errors:

5. The right solution

The IsValid ISSN API handles format checking, MOD-11 checksum validation, and input normalisation in a single call. Pass any string and get back a definitive answer plus the canonical hyphenated form.

Full parameter reference and response schema: ISSN Validation API docs →

6. Python code example

from isvalid import create_client

iv = create_client(api_key=os.environ["ISVALID_API_KEY"])

# ── Example usage ────────────────────────────────────────────────────────────

result = iv.issn("0378-5955")

if not result["valid"]:
    print("Invalid ISSN")
else:
    print("Valid:", result["valid"])   # → True
    print("ISSN:", result["issn"])     # → '0378-5955'

# Validate ISSNs before importing journal records
def import_journals(rows: list[dict]) -> list[dict]:
    results = []

    for row in rows:
        if not row.get("issn"):
            results.append({**row, "issn_status": "missing"})
            continue

        check = iv.issn(row["issn"])

        if not check["valid"]:
            results.append({**row, "issn_status": "invalid"})
            continue

        results.append({
            **row,
            "issn": check["issn"],          # canonical XXXX-XXXX format
            "issn_status": "valid",
        })

    return results

7. cURL example

Validate an ISSN:

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/issn?value=0378-5955"

Without hyphen:

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/issn?value=03785955"

ISSN with X check digit:

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/issn?value=0317-847X"

Invalid ISSN (bad check digit):

curl -H "Authorization: Bearer YOUR_API_KEY" \
  "https://api.isvalid.dev/v0/issn?value=0378-5950"

8. Understanding the response

Valid ISSN:

{
  "valid": true,
  "issn": "0378-5955"
}

Valid ISSN with X check digit:

{
  "valid": true,
  "issn": "0317-847X"
}

Invalid ISSN:

{
  "valid": false
}

9. Edge cases

# ISSN-L is validated the same way as a regular ISSN
result = iv.issn("1476-4687")  # Nature (linking ISSN)
print(result["valid"])  # → True
print(result["issn"])   # → '1476-4687'

# X check digit — perfectly valid
result = iv.issn("0317-847X")
print(result["valid"])  # → True
print(result["issn"])   # → '0317-847X'

# lowercase x is also accepted
result2 = iv.issn("0317-847x")
print(result2["valid"])  # → True
print(result2["issn"])   # → '0317-847X'

10. Summary