.. highlight:: cython

Unicode and passing strings
===========================

Similar to the string semantics in Python 3, Cython also strictly
separates byte strings and unicode strings.  Above all, this means
that there is no automatic conversion between byte strings and unicode
strings (except for what Python 2 does in string operations).  All
encoding and decoding must pass through an explicit encoding/decoding
step.

General notes about C strings
-----------------------------

In many use cases, C strings (a.k.a. character pointers) are slow
and cumbersome.  For one, they usually require manual memory
management in one way or another, which makes it more likely to
introduce bugs into your code.

Then, Python string objects cache their length, so requesting it
(e.g. to validate the bounds of index access or when concatenating
two strings into one) is an efficient constant time operation.
In contrast, calling :c:func:`strlen()` to get this information
from a C string takes linear time, which makes many operations on
C strings rather costly.

Regarding text processing, Python has built-in support for Unicode,
which C lacks completely.  If you are dealing with Unicode text,
you are usually better off using Python Unicode string objects than
trying to work with encoded data in C strings.  Cython makes this
quite easy and efficient.

Generally speaking: unless you know what you are doing, avoid
using C strings where possible and use Python string objects instead.
The obvious exception to this is when passing them back and forth
from and to external C code.  Also, C++ strings remember their length
as well, so they can provide a suitable alternative to Python bytes
objects in some cases.

Passing byte strings
--------------------

It is very easy to pass byte strings between C code and Python.
When receiving a byte string from a C library, you can let Cython
convert it into a Python byte string by simply assigning it to a
Python variable::

    cdef char* c_string = c_call_returning_a_c_string()
    cdef bytes py_string = c_string

A type cast to ``object`` or ``bytes`` will do the same thing::

    py_string = <bytes> c_string

This creates a Python byte string object that holds a copy of the
original C string.  It can be safely passed around in Python code, and
will be garbage collected when the last reference to it goes out of
scope.  It is important to remember that null bytes in the string act
as terminator character, as generally known from C.  The above will
therefore only work correctly for C strings that do not contain null
bytes.

Besides not working for null bytes, the above is also very inefficient
for long strings, since Cython has to call :c:func:`strlen()` on the
C string first to find out the length by counting the bytes up to the
terminating null byte.  In many cases, the user code will know the
length already, e.g. because a C function returned it.  In this case,
it is much more efficient to tell Cython the exact number of bytes by
slicing the C string::

    cdef char* c_string = NULL
    cdef Py_ssize_t length = 0

    # get pointer and length from a C function
    get_a_c_string(&c_string, &length)

    py_bytes_string = c_string[:length]

Here, no additional byte counting is required and ``length`` bytes from
the ``c_string`` will be copied into the Python bytes object, including
any null bytes.  Keep in mind that the slice indices are assumed to be
accurate in this case and no bounds checking is done, so incorrect
slice indices will lead to data corruption and crashes.

Note that the creation of the Python bytes string can fail with an
exception, e.g. due to insufficient memory.  If you need to
:c:func:`free()` the string after the conversion, you should wrap
the assignment in a try-finally construct::

    cimport stdlib
    cdef bytes py_string
    cdef char* c_string = c_call_creating_a_new_c_string()
    try:
        py_string = c_string
    finally:
        stdlib.free(c_string)

To convert the byte string back into a C :c:type:`char*`, use the
opposite assignment::

    cdef char* other_c_string = py_string

This is a very fast operation after which ``other_c_string`` points to
the byte string buffer of the Python string itself.  It is tied to the
life time of the Python string.  When the Python string is garbage
collected, the pointer becomes invalid.  It is therefore important to
keep a reference to the Python string as long as the :c:type:`char*`
is in use.  Often enough, this only spans the call to a C function that
receives the pointer as parameter.  Special care must be taken,
however, when the C function stores the pointer for later use.  Apart
from keeping a Python reference to the string object, no manual memory
management is required.

Dealing with "const"
--------------------

Many C libraries use the ``const`` modifier in their API to declare
that they will not modify a string, or to require that users must
not modify a string they return, for example:

.. code-block:: c

    typedef const char specialChar;
    int process_string(const char* s);
    const unsigned char* look_up_cached_string(const unsigned char* key);

Cython does not currently have support for the ``const`` modifier in
the language, but it allows users to make the necessary declarations
at a textual level.

In general, for arguments of external C functions, the ``const``
modifier does not matter and can be left out in the Cython
declaration (e.g. in a .pxd file).  The C compiler will still do
the right thing, even if you declare this to Cython::

    cdef extern from "someheader.h":
        int process_string(char* s)   # note: looses API information!

However, in most other situations, such as for return values and
variables that use specifically typedef-ed API types, it does matter
and the C compiler will emit a warning if used incorrectly.  To help
with this, you can use the type definitions in the ``libc.string``
module, e.g.::

    from libc.string cimport const_char, const_uchar

    cdef extern from "someheader.h":
        ctypedef const_char specialChar
        int process_string(const_char* s)
        const_uchar* look_up_cached_string(const_uchar* key)

Note: even if the API only uses ``const`` for function arguments,
it is still preferable to properly declare them using the
:c:type:`const_char` types in order to simplify adaptations, e.g.
if Cython ever gains language support for ``const``.

Decoding bytes to text
----------------------

The initially presented way of passing and receiving C strings is
sufficient if your code only deals with binary data in the strings.
When we deal with encoded text, however, it is best practice to decode
the C byte strings to Python Unicode strings on reception, and to
encode Python Unicode strings to C byte strings on the way out.

With a Python byte string object, you would normally just call the
``.decode()`` method to decode it into a Unicode string::

    ustring = byte_string.decode('UTF-8')

Cython allows you to do the same for a C string, as long as it
contains no null bytes::

    cdef char* some_c_string = c_call_returning_a_c_string()
    ustring = some_c_string.decode('UTF-8')

And, more efficiently, for strings where the length is known::

    cdef char* c_string = NULL
    cdef Py_ssize_t length = 0

    # get pointer and length from a C function
    get_a_c_string(&c_string, &length)

    ustring = c_string[:length].decode('UTF-8')

The same should be used when the string contains null bytes, e.g. when
it uses an encoding like UCS-4, where each character is encoded in four
bytes most of which tend to be 0.

Again, no bounds checking is done if slice indices are provided, so
incorrect indices lead to data corruption and crashes.  However, using
negative indices is possible since Cython 0.17 and will inject a call
to :c:func:`strlen()` in order to determine the string length.
Obviously, this only works for 0-terminated strings without internal
null bytes.  Text encoded in UTF-8 or one of the ISO-8859 encodings is
usually a good candidate.  If in doubt, it's better to pass indices
that are 'obviously' correct than to rely on the data to be as expected.

It is common practice to wrap string conversions (and non-trivial type
conversions in general) in dedicated functions, as this needs to be
done in exactly the same way whenever receiving text from C.  This
could look as follows::

    cimport python_unicode
    cimport stdlib

    cdef unicode tounicode(char* s):
        return s.decode('UTF-8', 'strict')

    cdef unicode tounicode_with_length(
            char* s, size_t length):
        return s[:length].decode('UTF-8', 'strict')

    cdef unicode tounicode_with_length_and_free(
            char* s, size_t length):
        try:
            return s[:length].decode('UTF-8', 'strict')
        finally:
            stdlib.free(s)

Most likely, you will prefer shorter function names in your code based
on the kind of string being handled.  Different types of content often
imply different ways of handling them on reception.  To make the code
more readable and to anticipate future changes, it is good practice to
use separate conversion functions for different types of strings.

Encoding text to bytes
----------------------

The reverse way, converting a Python unicode string to a C
:c:type:`char*`, is pretty efficient by itself, assuming that what
you actually want is a memory managed byte string::

    py_byte_string = py_unicode_string.encode('UTF-8')
    cdef char* c_string = py_byte_string

As noted before, this takes the pointer to the byte buffer of the
Python byte string.  Trying to do the same without keeping a reference
to the Python byte string will fail with a compile error::

    # this will not compile !
    cdef char* c_string = py_unicode_string.encode('UTF-8')

Here, the Cython compiler notices that the code takes a pointer to a
temporary string result that will be garbage collected after the
assignment.  Later access to the invalidated pointer will read invalid
memory and likely result in a segfault.  Cython will therefore refuse
to compile this code.

C++ strings
-----------

When wrapping a C++ library, strings will usually come in the form of
the :c:type:`std::string` class.  As with C strings, Python byte strings
automatically coerce from and to C++ strings::

    # distutils: language = c++

    from libcpp.string cimport string

    cdef string s = py_bytes_object
    try:
        s.append('abc')
        py_bytes_object = s
    finally:
        del s

The memory management situation is different than in C because the
creation of a C++ string makes an independent copy of the string
buffer which the string object then owns.  It is therefore possible
to convert temporarily created Python objects directly into C++
strings.  A common way to make use of this is when encoding a Python
unicode string into a C++ string::

    cdef string cpp_string = py_unicode_string.encode('UTF-8')

Note that this involves a bit of overhead because it first encodes
the Unicode string into a temporarily created Python bytes object
and then copies its buffer into a new C++ string.

For the other direction, efficient decoding support is available
in Cython 0.17 and later::

    cdef string s = string('abcdefg')

    ustring1 = s.decode('UTF-8')
    ustring2 = s[2:-2].decode('UTF-8')

For C++ strings, decoding slices will always take the proper length
of the string into account and apply Python slicing semantics (e.g.
return empty strings for out-of-bounds indices).

Source code encoding
--------------------

When string literals appear in the code, the source code encoding is
important.  It determines the byte sequence that Cython will store in
the C code for bytes literals, and the Unicode code points that Cython
builds for unicode literals when parsing the byte encoded source file.
Following `PEP 263`_, Cython supports the explicit declaration of
source file encodings.  For example, putting the following comment at
the top of an ``ISO-8859-15`` (Latin-9) encoded source file (into the
first or second line) is required to enable ``ISO-8859-15`` decoding
in the parser::

    # -*- coding: ISO-8859-15 -*-

When no explicit encoding declaration is provided, the source code is
parsed as UTF-8 encoded text, as specified by `PEP 3120`_.  `UTF-8`_
is a very common encoding that can represent the entire Unicode set of
characters and is compatible with plain ASCII encoded text that it
encodes efficiently.  This makes it a very good choice for source code
files which usually consist mostly of ASCII characters.

.. _`PEP 263`: http://www.python.org/dev/peps/pep-0263/
.. _`PEP 3120`: http://www.python.org/dev/peps/pep-3120/
.. _`UTF-8`: http://en.wikipedia.org/wiki/UTF-8

As an example, putting the following line into a UTF-8 encoded source
file will print ``5``, as UTF-8 encodes the letter ``'ö'`` in the two
byte sequence ``'\xc3\xb6'``::

    print( len(b'abcö') )

whereas the following ``ISO-8859-15`` encoded source file will print
``4``, as the encoding uses only 1 byte for this letter::

    # -*- coding: ISO-8859-15 -*-
    print( len(b'abcö') )

Note that the unicode literal ``u'abcö'`` is a correctly decoded four
character Unicode string in both cases, whereas the unprefixed Python
``str`` literal ``'abcö'`` will become a byte string in Python 2 (thus
having length 4 or 5 in the examples above), and a 4 character Unicode
string in Python 3.  If you are not familiar with encodings, this may
not appear obvious at first read.  See `CEP 108`_ for details.

As a rule of thumb, it is best to avoid unprefixed non-ASCII ``str``
literals and to use unicode string literals for all text.  Cython also
supports the ``__future__`` import ``unicode_literals`` that instructs
the parser to read all unprefixed ``str`` literals in a source file as
unicode string literals, just like Python 3.

.. _`CEP 108`: http://wiki.cython.org/enhancements/stringliterals

Single bytes and characters
---------------------------

The Python C-API uses the normal C :c:type:`char` type to represent
a byte value, but it has two special integer types for a Unicode code
point value, i.e. a single Unicode character: :c:type:`Py_UNICODE`
and :c:type:`Py_UCS4`.  Since version 0.13, Cython supports the
first natively, support for :c:type:`Py_UCS4` is new in Cython 0.15.
:c:type:`Py_UNICODE` is either defined as an unsigned 2-byte or
4-byte integer, or as :c:type:`wchar_t`, depending on the platform.
The exact type is a compile time option in the build of the CPython
interpreter and extension modules inherit this definition at C
compile time.  The advantage of :c:type:`Py_UCS4` is that it is
guaranteed to be large enough for any Unicode code point value,
regardless of the platform.  It is defined as a 32bit unsigned int
or long.

In Cython, the :c:type:`char` type behaves differently from the
:c:type:`Py_UNICODE` and :c:type:`Py_UCS4` types when coercing
to Python objects.  Similar to the behaviour of the bytes type in
Python 3, the :c:type:`char` type coerces to a Python integer
value by default, so that the following prints 65 and not ``A``::

    # -*- coding: ASCII -*-

    cdef char char_val = 'A'
    assert char_val == 65   # ASCII encoded byte value of 'A'
    print( char_val )

If you want a Python bytes string instead, you have to request it
explicitly, and the following will print ``A`` (or ``b'A'`` in Python
3)::

    print( <bytes>char_val )

The explicit coercion works for any C integer type.  Values outside of
the range of a :c:type:`char` or :c:type:`unsigned char` will raise an
``OverflowError`` at runtime.  Coercion will also happen automatically
when assigning to a typed variable, e.g.::

    cdef bytes py_byte_string
    py_byte_string = char_val

On the other hand, the :c:type:`Py_UNICODE` and :c:type:`Py_UCS4`
types are rarely used outside of the context of a Python unicode string,
so their default behaviour is to coerce to a Python unicode object.  The
following will therefore print the character ``A``, as would the same
code with the :c:type:`Py_UNICODE` type::

    cdef Py_UCS4 uchar_val = u'A'
    assert uchar_val == 65 # character point value of u'A'
    print( uchar_val )

Again, explicit casting will allow users to override this behaviour.
The following will print 65::

    cdef Py_UCS4 uchar_val = u'A'
    print( <long>uchar_val )

Note that casting to a C ``long`` (or ``unsigned long``) will work
just fine, as the maximum code point value that a Unicode character
can have is 1114111 (``0x10FFFF``).  On platforms with 32bit or more,
``int`` is just as good.


Narrow Unicode builds
----------------------

In narrow Unicode builds of CPython before version 3.3, i.e. builds
where ``sys.maxunicode`` is 65535 (such as all Windows builds, as
opposed to 1114111 in wide builds), it is still possible to use
Unicode character code points that do not fit into the 16 bit wide
:c:type:`Py_UNICODE` type.  For example, such a CPython build will
accept the unicode literal ``u'\U00012345'``.  However, the
underlying system level encoding leaks into Python space in this
case, so that the length of this literal becomes 2 instead of 1.
This also shows when iterating over it or when indexing into it.
The visible substrings are ``u'\uD808'`` and ``u'\uDF45'`` in this
example.  They form a so-called surrogate pair that represents the
above character.

For more information on this topic, it is worth reading the `Wikipedia
article about the UTF-16 encoding`_.

.. _`Wikipedia article about the UTF-16 encoding`: http://en.wikipedia.org/wiki/UTF-16/UCS-2

The same properties apply to Cython code that gets compiled for a
narrow CPython runtime environment.  In most cases, e.g. when
searching for a substring, this difference can be ignored as both the
text and the substring will contain the surrogates.  So most Unicode
processing code will work correctly also on narrow builds.  Encoding,
decoding and printing will work as expected, so that the above literal
turns into exactly the same byte sequence on both narrow and wide
Unicode platforms.

However, programmers should be aware that a single :c:type:`Py_UNICODE`
value (or single 'character' unicode string in CPython) may not be
enough to represent a complete Unicode character on narrow platforms.
For example, if an independent search for ``u'\uD808'`` and
``u'\uDF45'`` in a unicode string succeeds, this does not necessarily
mean that the character ``u'\U00012345`` is part of that string.  It
may well be that two different characters are in the string that just
happen to share a code unit with the surrogate pair of the character
in question.  Looking for substrings works correctly because the two
code units in the surrogate pair use distinct value ranges, so the
pair is always identifiable in a sequence of code points.

As of version 0.15, Cython has extended support for surrogate pairs so
that you can safely use an ``in`` test to search character values from
the full :c:type:`Py_UCS4` range even on narrow platforms::

    cdef Py_UCS4 uchar = 0x12345
    print( uchar in some_unicode_string )

Similarly, it can coerce a one character string with a high Unicode
code point value to a Py_UCS4 value on both narrow and wide Unicode
platforms::

    cdef Py_UCS4 uchar = u'\U00012345'
    assert uchar == 0x12345

In CPython 3.3 and later, the :c:type:`Py_UNICODE` type is an alias
for the system specific :c:type:`wchar_t` type and is no longer tied
to the internal representation of the Unicode string.  Instead, any
Unicode character can be represented on all platforms without
resorting to surrogate pairs.  This implies that narrow builds no
longer exist from that version on, regardless of the size of
:c:type:`Py_UNICODE`.  See
`PEP 393 <http://www.python.org/dev/peps/pep-0393/>`_  for details.

Cython 0.16 and later handles this change internally and does the right
thing also for single character values as long as either type inference
is applied to untyped variables or the portable :c:type:`Py_UCS4` type
is explicitly used in the source code instead of the platform specific
:c:type:`Py_UNICODE` type.  Optimisations that Cython applies to the
Python unicode type will automatically adapt to PEP 393 at C compile
time, as usual.

Iteration
---------

Cython 0.13 supports efficient iteration over :c:type:`char*`,
bytes and unicode strings, as long as the loop variable is
appropriately typed. So the following will generate the expected
C code::

    cdef char* c_string = ...

    cdef char c
    for c in c_string[:100]:
        if c == 'A': ...

The same applies to bytes objects::

    cdef bytes bytes_string = ...

    cdef char c
    for c in bytes_string:
        if c == 'A': ...

For unicode objects, Cython will automatically infer the type of the
loop variable as :c:type:`Py_UCS4`::

    cdef unicode ustring = ...

    # NOTE: no typing required for 'uchar' !
    for uchar in ustring:
        if uchar == u'A': ...

The automatic type inference usually leads to much more efficient code
here.  However, note that some unicode operations still require the
value to be a Python object, so Cython may end up generating redundant
conversion code for the loop variable value inside of the loop.  If
this leads to a performance degradation for a specific piece of code,
you can either type the loop variable as a Python object explicitly,
or assign its value to a Python typed variable somewhere inside of the
loop to enforce one-time coercion before running Python operations on
it.

There are also optimisations for ``in`` tests, so that the following
code will run in plain C code, (actually using a switch statement)::

    cdef Py_UCS4 uchar_val = get_a_unicode_character()
    if uchar_val in u'abcABCxY':
        ...

Combined with the looping optimisation above, this can result in very
efficient character switching code, e.g. in unicode parsers.
