SWIG and Python3 unicode

Anyone familiar with Python probably knew its history of Unicode support. If you add Python3, Unicode, and SWIG together, imagine what might go wrong?

Python3, Unicode, SWIG, and me

I was debugging a test failure written in Python just now and it is failing with this error:

Many of the end-to-end tests here are written in Python because they are convenient - no one wants to write a C++ code to drive MySql and our infra service to do a series of stuff.

UnicodeEncodeError: 'latin-1' codec can't encode character '\udcfa' in position 293: ordinal not in range(256)

The code looks like this:

sql = get_sql_from_some_magic_place()
decoded_sql = cUnescape(sql.decode("latin-1"))
decoded_sql_str = decoded_sql.encode("latin-1")
execute(decoded_sql_str)

The code seems straight-forward enough. The offending string looks like this: b"SELECT from blah WHERE col='\\372'.

This string was originally escaped by folly::cEscape which does simple thing rather simple - converts the string to be a C representation where ‘' are double escaped and any non-printable characters are escaped with octal. This is convenient as those escaped strings are safe to pass around without worry for encoding as they are, well, ASCII.

folly is Facebook’s open source standard C++ library collection. See https://github.com/facebook/folly for more information.

It is convenient, until you need to call from Python, for which you’ll need to use SWIG:

If you don’t know SWIG - just think it’s a tool that generates Python wrapper for C++ code so that they can be called from Python code. In this case, folly::cUnescape. Go to http://www.swig.org/ to learn more. Many language have equivalent tool/feature built-in, P/invoke in C#, cgo in go, JNI in Java, etc.

std::string cUnescape(const std::string& a) {
  std::string b;
  folly::cUnescape(a, b);
  return b;
}

I was scratching my ahead trying to understand what is happening as there is no way the strings are converted to ‘\udcfa’, until I realize cUnescape might be at fault.

It turns out, SWIG expects UTF-8 string and returns UTF-8 strings back. “\372” can be converted to UTF-8 without any trouble, but once it is unescaped it becomes “\372” which is 0xfa that is going to be interpreted as UTF-8:

b"\372".decode("utf-8", errors="surrogateescape").encode("latin-1")

And you get:

UnicodeEncodeError: 'latin-1' codec can't encode character '\udcfa' in position 0: ordinal not in range(256)

The fix

To fix the problem, you can encode the buffer again with surrogateescape:

>>> b"\372".decode("utf-8", errors="surrogateescape").encode("utf-8", errors="surrogateescape").decode("latin-1")
'ú'

Seems rather backwards, isn’t it? Why not just stop messing with the strings?

That’s exactly what was discussed in SWIG doc here: http://www.swig.org/Doc4.0/Python.html#Python_nn77. There is a magic macro you can use:

%module char_to_bytes
%begin %{
#define SWIG_PYTHON_STRICT_BYTE_CHAR
%}
std::string cUnescape(const std::string& a) {
  std::string b;
  folly::cUnescape(a, b);
  return b;
}

And the original code can be changed to:

sql = get_sql_from_some_magic_place()
decoded_sql = cUnescape(sql).decode("latin-1")
execute(decoded_sql)

Much simpler too.

I’m just happy that I mostly write C++ instead of Python…


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