Unicode and passing strings

Cython 0.19.1

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 by default 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. For simple cases, the module-level c_string_type and c_string_encoding directives can be used to implicitly insert these encoding/decoding steps to ease conversion between Python and C strings.

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 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 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 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()
    py_string = c_string

To convert the byte string back into a C 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 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:

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

Since version 0.18, Cython has support for the const modifier in the language, so you can declare the above functions straight away as follows:

cdef extern from "someheader.h":
    ctypedef const char specialChar
    int process_string(const char* s)
    const unsigned char* look_up_cached_string(const unsigned char* key)

Previous versions required users to make the necessary declarations at a textual level. If you need to support older Cython versions, you can use the following approach.

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 at least 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 these provided const_char types in order to simplify adaptations. In Cython 0.18, these standard declarations have been changed to use the correct const modifier, so your code will automatically benefit from the new const support if it uses them.

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 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):
        return s[:length].decode('UTF-8', 'strict')

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 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 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
    py_bytes_object = s
    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).

Auto encoding and decoding

Cython 0.19 comes with two new directives: c_string_type and c_string_encoding. They can be used to change the Python string types that C/C++ strings coerce from and to. By default, they only coerce from and to the bytes type, and encoding or decoding must be done explicitly, as described above.

There are two use cases where this is inconvenient. First, if all C strings that are being processed (or the large majority) contain text, automatic encoding and decoding from and to Python unicode objects can reduce the code overhead a little. In this case, you can set the c_string_type directive in your module to unicode and the c_string_encoding to the encoding that your C code uses, for example:

# cython: c_string_type=unicode, c_string_encoding=utf8

cdef char* c_string = 'abcdefg'

# implicit decoding:
cdef object py_unicode_object = c_string

# explicit conversion to Python bytes:
py_bytes_object = <bytes>c_string

The second use case is when all C strings that are being processed only contain ASCII encodable characters (e.g. numbers) and you want your code to use the native legacy string type in Python 2 for them, instead of always using Unicode. In this case, you can set the string type to str:

# cython: c_string_type=str, c_string_encoding=ascii

cdef char* c_string = 'abcdefg'

# implicit decoding in Py3, bytes conversion in Py2:
cdef object py_str_object = c_string

# explicit conversion to Python bytes:
py_bytes_object = <bytes>c_string

# explicit conversion to Python unicode:
py_bytes_object = <unicode>c_string

The other direction, i.e. automatic encoding to C strings, is only supported for the ASCII codec (and the “default encoding”, which is runtime specific and may or may not be ASCII). This is because CPython handles the memory management in this case by keeping an encoded copy of the string alive together with the original unicode string. Otherwise, there would be no way to limit the lifetime of the encoded string in any sensible way, thus rendering any attempt to extract a C string pointer from it a dangerous endeavour. As long as you stick to the ASCII encoding for the c_string_encoding directive, though, the following will work:

# cython: c_string_type=unicode, c_string_encoding=ascii

def func():
    ustring = u'abc'
    cdef char* s = ustring
    return s[0]    # returns u'a'

(This example uses a function context in order to safely control the lifetime of the Unicode string. Global Python variables can be modified from the outside, which makes it dangerous to rely on the lifetime of their values.)

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.

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.

Single bytes and characters

The Python C-API uses the normal C 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: Py_UNICODE and Py_UCS4. Since version 0.13, Cython supports the first natively, support for Py_UCS4 is new in Cython 0.15. Py_UNICODE is either defined as an unsigned 2-byte or 4-byte integer, or as 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 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 char type behaves differently from the Py_UNICODE and Py_UCS4 types when coercing to Python objects. Similar to the behaviour of the bytes type in Python 3, the 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 char or 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 Py_UNICODE and 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 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 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.

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 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 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 Py_UNICODE type is an alias for the system specific 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 Py_UNICODE. See PEP 393 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 Py_UCS4 type is explicitly used in the source code instead of the platform specific Py_UNICODE type. Optimisations that Cython applies to the Python unicode type will automatically adapt to PEP 393 at C compile time, as usual.


Cython 0.13 supports efficient iteration over 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 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.

Windows and wide character APIs

Windows system APIs natively support Unicode in the form of zero-terminated UTF-16 encoded wchar_t* strings, so called “wide strings”.

By default, Windows builds of CPython define Py_UNICODE as a synonym for wchar_t. This makes internal unicode representation compatible with UTF-16 and allows for efficient zero-copy conversions. This also means that Windows builds are always Narrow Unicode builds with all the caveats.

To aid interoperation with Windows APIs, Cython 0.19 supports wide strings (in the form of Py_UNICODE*) and implicitly converts them to and from unicode string objects. These conversions behave the same way as they do for char* and bytes as described in Passing byte strings.

In addition to automatic conversion, unicode literals that appear in C context become C-level wide string literals and len() built-in function is specialized to compute the length of zero-terminated Py_UNICODE* string or array.

Here is an example of how one would call a Unicode API on Windows:

cdef extern from "Windows.h":

    ctypedef Py_UNICODE WCHAR
    ctypedef const WCHAR* LPCWSTR
    ctypedef void* HWND

    int MessageBoxW(HWND hWnd, LPCWSTR lpText, LPCWSTR lpCaption, int uType)

title = u"Windows Interop Demo - Python %d.%d.%d" % sys.version_info[:3]
MessageBoxW(NULL, u"Hello Cython \u263a", title, 0)


The use of Py_UNICODE* strings outside of Windows is strongly discouraged. Py_UNICODE is inherently not portable between different platforms and Python versions.

CPython 3.3 has moved to a flexible internal representation of unicode strings (PEP 393), making all Py_UNICODE related APIs deprecated and inefficient.

One consequence of CPython 3.3 changes is that len() of unicode strings is always measured in code points (“characters”), while Windows API expect the number of UTF-16 code units (where each surrogate is counted individually). To always get the number of code units, call PyUnicode_GetSize() directly.