6.6. "stringprep" — Internet String Preparation
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**Source code:** Lib/stringprep.py

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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”).
