Applying Stringprep

Python
import stringprep
import unicodedata

def prepare_string(s, mapping_table=stringprep.b1_set):
    """Apply basic stringprep steps: map, then check prohibited chars."""
    # Step 1: Mapping - remove characters mapped to nothing (B.1)
    chars = [c for c in s if c not in mapping_table]

    # Step 2: Normalize (KC form, per RFC 3454 profiles like Nameprep)
    normalized = unicodedata.normalize('NFKC', ''.join(chars))

    # Step 3: Prohibit check
    for c in normalized:
        if stringprep.in_table_c12(c):  # Non-ASCII space
            raise ValueError(f"Prohibited character: {c!r}")

    return normalized

print(prepare_string("Hello\u00ad World"))  # soft hyphen (B.1) is stripped

Note: stringprep is deprecated (removal planned per PEP 594-style cleanup for legacy protocol modules). Only use it for interoperability with existing systems; do not build new protocols on it. Prefer modern Unicode normalization (unicodedata, IDNA via encodings.idna or the third-party idna package) for anything new.

Stringprep implements RFC 3454's generic framework. A concrete profile (e.g., Nameprep, SASLprep) picks specific tables/operations from this toolkit. To implement or verify a profile:

Progress:

• Step 1: Identify the profile spec (Nameprep RFC 3491, SASLprep RFC 4013, etc.) and which tables it references
• Step 2: Apply the mapping step using the relevant in_table_* functions that indicate "map to nothing" or "map to space"
• Step 3: Apply normalization (usually NFKC) if the profile requires it
• Step 4: Run the prohibited output check — reject strings containing characters in prohibited tables
• Step 5: Run the bidirectional (BiDi) check — enforce RFC 3454 Section 6 rules on RandALCat/LCat mixing
• Step 6: Return the prepared string, or raise/report the specific violation

Key table functions