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Strings

{"type": "string"} generates a random string. Without any constraints, Faker's default text() generator is used.

Length constraints

minLength / maxLength

{"type": "string", "minLength": 6, "maxLength": 12}

'rgYiwK'

Strategy: generates a random alphanumeric string between minLength and maxLength characters.

Pattern

pattern

{"type": "string", "pattern": "^[A-Z]{3}-[0-9]{4}$"}

'LLL-5683'

Strategy: uses Hypothesis st.from_regex to generate a string matching the regex. JSON Schema regexes are JavaScript-compatible (ECMA 262) so we have a custom translator before handing it off to the Hypothesis (Python) generator. Matching rules follow JS; without ^...$ anchors it matches as a substring.

Limitations:

  • pattern and format are mutually exclusive — pattern takes precedence when both are specified.
  • For unanchored patterns (no $ end anchor), short regex matches are automatically padded with random characters to meet minLength, since JSON Schema pattern uses substring matching (re.search).
  • Anchored patterns (^...$) with very tight minLength/maxLength constraints may exhaust the search budget and raise NoExampleFoundError.

Format

format

{"type": "string", "format": "date"}

The format keyword selects a specific value generator. All supported formats are listed below.

Format Example Notes
date '2018-12-12' ISO 8601 date, always 10 characters
date-time '1989-09-22T15:57:18.372797+00:14' ISO 8601 datetime with timezone
time '04:36:05+02:00' RFC 3339 §5.6 full-time with UTC offset
duration 'P5Y6M2DT21H58M21S' ISO 8601 / RFC 3339 Appendix A duration
email 'christinaturner@example.net' ASCII email address
idn-email 'clarence34@example.org' Internationalized email (RFC 6531); ASCII subset
uuid '2d06e8cf-3805-4907-acd6-6193c7468f59' UUID v4, always 36 characters
uri 'https://manning.info/posts/list/taglogin.jsp' Absolute URI
uri-reference '/ozuI/LRVYBVE/f/hyctlqN' Absolute URI or relative reference path
uri-template 'http://www.peterson.com/categories/tagshome.asp{XHRKw}' RFC 6570 URI Template
iri 'https://www.turner.net/categorymain.php' IRI (RFC 3987); ASCII URIs are valid IRIs
iri-reference '/Iijda/JybooSdm' IRI-reference; reuses uri-reference logic
hostname 'db-75.cook-santiago.com' DNS hostname
idn-hostname 'desktop-77.johnson.org' Internationalized hostname (RFC 5890); ASCII subset
ipv4 '216.215.4.203' IPv4 address
ipv6 '89e0:6ab3:7250:ee18:260a:5963:dd81:b7f5' IPv6 address
json-pointer '/Svqy/uIQZzbA/TTwBxL/TYdOnGu' RFC 6901 JSON Pointer
relative-json-pointer '4/RkuzmDDZ/bRuaS/M' Relative JSON Pointer
regex '[A-Z][a-z]*' A valid regular expression pattern
password 'ZZMda($#z(5RGF...' A strong password string
byte 'bVJmbDFkdUFSUVhr...' Base64-encoded string
binary '\u00f6\u00cc\f6...' Binary data returned as a Python str

Strategy: each format maps to a dedicated generator method. Most variable-length formats (email, uri, hostname, duration, json-pointer, etc.) are length-aware — they accept minLength/maxLength and produce output in that range directly, without retry. If the requested length falls outside the format's valid bounds (e.g. minLength: 300 for email, which exceeds the RFC 5321 maximum of 254), an UnsatisfiableConstraintsError is raised. Fixed-length formats (date, uuid, etc.) ignore length constraints. The regex format and any unknown format resolved via Faker fall back to retry sampling (up to max_search attempts).

Limitations:

  • pattern takes precedence over format when both are set.
  • binary uses a Latin-1 decoded string so each byte value maps losslessly onto one code point. This keeps the JSON Schema type as string, but the result may contain non-printable characters.
  • contentMediaType is accepted in the schema but ignored during generation (it serves as a hint only).
  • password with maxLength < 4 produces random alphanumeric strings instead of strong passwords (Faker's password() generator requires at least 4 characters for its character-class guarantees).
  • duration generates realistic calendar values (years <= 10, months <= 11, etc.) for typical lengths (up to 20 chars). When minLength exceeds 20, the fallback inflates component values beyond realistic ranges to reach the target. Maximum achievable length is 32 characters; minLength > 32 raises UnsatisfiableConstraintsError.

Content encoding

contentEncoding

All RFC-defined contentEncoding values are supported. All return Python str values so the generated instance still conforms to JSON Schema type: string. Unknown values fall through to plain string generation.

Value RFC Description
"base64" RFC 4648 §4 64-character alphabet; encoded length is a multiple of 4
"base32" RFC 4648 §6 32-character alphabet; encoded length is a multiple of 8
"base16" RFC 4648 §8 Hex alphabet; encoded length is always even
"7bit" RFC 2045 §2.7 Printable ASCII (0x20–0x7E); no NUL or bare CR/LF
"8bit" RFC 2045 §2.8 Printable ASCII + octets >127; no NUL or bare CR/LF
"binary" RFC 2045 §2.9 Any octet sequence
"quoted-printable" RFC 2045 §6.7 Printable ASCII encoded per QP rules 
{"type": "string", "contentEncoding": "base64"}

'dGVzdA=='

{"type": "string", "contentEncoding": "base32"}

'ORSXG5BR'

{"type": "string", "contentEncoding": "base16"}

'48656C6C6F'

{"type": "string", "contentEncoding": "quoted-printable"}

'Hello World'

Strategy: minLength/maxLength constrain the encoded output length. For base64, base32, and base16, the generator first chooses a valid encoded length and then generates raw bytes that encode to that length before decoding the encoded result back to text.

Limitations: binary and 8bit may contain non-printable characters because they preserve arbitrary octets inside a Python string. For base64, base32, and base16, an UnsatisfiableConstraintsError is raised if no valid encoded length can fit within the requested range.