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# 6.6. `stringprep` — Internet String Preparation

When identifying things (such as host names) in the internet, it is often necessary to compare such identifications for “equality”. Exactly how this comparison is executed may depend on the application domain, e.g. whether it should be case-insensitive or not. It may be also necessary to restrict the possible identifications, to allow only identifications consisting of “printable” characters.

RFC 3454 defines a procedure for “preparing” Unicode strings in internet protocols. Before passing strings onto the wire, they are processed with the preparation procedure, after which they have a certain normalized form. The RFC defines a set of tables, which can be combined into profiles. Each profile must define which tables it uses, and what other optional parts of the `stringprep` procedure are part of the profile. One example of a `stringprep` profile is `nameprep`, which is used for internationalized domain names.

The module `stringprep` only exposes the tables from RFC 3454. As these tables would be very large to represent them as dictionaries or lists, the module uses the Unicode character database internally. The module source code itself was generated using the `mkstringprep.py` utility.

As a result, these tables are exposed as functions, not as data structures. There are two kinds of tables in the RFC: sets and mappings. For a set, `stringprep` provides the “characteristic function”, i.e. a function that returns true if the parameter is part of the set. For mappings, it provides the mapping function: given the key, it returns the associated value. Below is a list of all functions available in the module.

`stringprep.``in_table_a1`(code)

  Determine whether code is in tableA.1 (Unassigned code points in Unicode 3.2).

`stringprep.``in_table_b1`(code)

  Determine whether code is in tableB.1 (Commonly mapped to nothing).

`stringprep.``map_table_b2`(code)

  Return the mapped value for code according to tableB.2 (Mapping for case-folding used with NFKC).

`stringprep.``map_table_b3`(code)

  Return the mapped value for code according to tableB.3 (Mapping for case-folding used with no normalization).

`stringprep.``in_table_c11`(code)

  Determine whether code is in tableC.1.1 (ASCII space characters).

`stringprep.``in_table_c12`(code)

  Determine whether code is in tableC.1.2 (Non-ASCII space characters).

`stringprep.``in_table_c11_c12`(code)

  Determine whether code is in tableC.1 (Space characters, union of C.1.1 and C.1.2).

`stringprep.``in_table_c21`(code)

  Determine whether code is in tableC.2.1 (ASCII control characters).

`stringprep.``in_table_c22`(code)

  Determine whether code is in tableC.2.2 (Non-ASCII control characters).

`stringprep.``in_table_c21_c22`(code)

  Determine whether code is in tableC.2 (Control characters, union of C.2.1 and C.2.2).

`stringprep.``in_table_c3`(code)

  Determine whether code is in tableC.3 (Private use).

`stringprep.``in_table_c4`(code)

  Determine whether code is in tableC.4 (Non-character code points).

`stringprep.``in_table_c5`(code)

  Determine whether code is in tableC.5 (Surrogate codes).

`stringprep.``in_table_c6`(code)

  Determine whether code is in tableC.6 (Inappropriate for plain text).

`stringprep.``in_table_c7`(code)

  Determine whether code is in tableC.7 (Inappropriate for canonical representation).

`stringprep.``in_table_c8`(code)

  Determine whether code is in tableC.8 (Change display properties or are deprecated).

`stringprep.``in_table_c9`(code)

  Determine whether code is in tableC.9 (Tagging characters).

`stringprep.``in_table_d1`(code)

  Determine whether code is in tableD.1 (Characters with bidirectional property “R” or “AL”).

`stringprep.``in_table_d2`(code)

  Determine whether code is in tableD.2 (Characters with bidirectional property “L”).

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