Allow writing generated code to a file-like object. (#115)

This allows a caller to better control when and where the generated code is written, for issue #47.

doc/source/cdef.rst

@@ -667,6 +667,12 @@ write).  If you choose, you can include this .py file pre-packaged in
 your own distributions: it is identical for any Python version (2 or
 3).
 
+*New in version 1.17.1:* ``filename`` can instead be a file-like object
+(such as a StringIO instance). The generated code will be written to this
+file-like object. However, if an error arises during generation, partial
+code may be written; it is the caller's responsibility to clean up
+if this occurs.
+
 **ffibuilder.emit_c_code(filename):** generate the given .c file (for API
 mode) without compiling it.  Can be used if you have some other method
 to compile it, e.g. if you want to integrate with some larger build
@@ -676,6 +682,12 @@ platform, the .c file itself is generic (it would be exactly the same
 if produced on a different OS, with a different version of CPython, or
 with PyPy; it is done with generating the appropriate ``#ifdef``).
 
+*New in version 1.17.1:* ``filename`` can instead be a file-like object
+(such as a StringIO instance). The generated code will be written to this
+file-like object. However, if an error arises during generation, partial
+code may be written; it is the caller's responsibility to clean up
+if this occurs.
+
 **ffibuilder.distutils_extension(tmpdir='build', verbose=True):** for
 distutils-based ``setup.py`` files.  Calling this creates the .c file
 if needed in the given ``tmpdir``, and returns a

src/cffi/recompiler.py

@@ -1419,13 +1419,20 @@ def write(self, s):
                 s = s.encode('ascii')
             super(NativeIO, self).write(s)
 
+def _is_file_like(maybefile):
+    # compare to xml.etree.ElementTree._get_writer
+    return hasattr(maybefile, 'write')
+
 def _make_c_or_py_source(ffi, module_name, preamble, target_file, verbose):
     if verbose:
         print("generating %s" % (target_file,))
     recompiler = Recompiler(ffi, module_name,
                             target_is_python=(preamble is None))
     recompiler.collect_type_table()
     recompiler.collect_step_tables()
+    if _is_file_like(target_file):
+        recompiler.write_source_to_f(target_file, preamble)
+        return True
     f = NativeIO()
     recompiler.write_source_to_f(f, preamble)
     output = f.getvalue()
@@ -1526,6 +1533,9 @@ def recompile(ffi, module_name, preamble, tmpdir='.', call_c_compiler=True,
     if ffi._windows_unicode:
         ffi._apply_windows_unicode(kwds)
     if preamble is not None:
+        if call_c_compiler and _is_file_like(c_file):
+            raise TypeError("Writing to file-like objects is not supported "
+                            "with call_c_compiler=True")
         embedding = (ffi._embedding is not None)
         if embedding:
             ffi._apply_embedding_fix(kwds)

testing/cffi1/test_new_ffi_1.py

@@ -4,7 +4,7 @@
 import cffi
 from testing.udir import udir
 from testing.support import *
-from cffi.recompiler import recompile
+from cffi.recompiler import recompile, NativeIO
 from cffi.cffi_opcode import PRIMITIVE_TO_INDEX
 
 SIZE_OF_INT   = ctypes.sizeof(ctypes.c_int)
@@ -1776,6 +1776,13 @@ def test_emit_c_code(self):
         ffi.emit_c_code(c_file)
         assert os.path.isfile(c_file)
 
+    def test_emit_c_code_to_file_obj(self):
+        ffi = cffi.FFI()
+        ffi.set_source("foobar", "??")
+        fileobj = NativeIO()
+        ffi.emit_c_code(fileobj)
+        assert 'foobar' in fileobj.getvalue()
+
     def test_import_from_lib(self):
         ffi2 = cffi.FFI()
         ffi2.cdef("int myfunc(int); extern int myvar;\n#define MYFOO ...\n")