GH-38857: [Python] Add append mode for pyarrow.OsFile (#38820)

### Rationale for this change

Seems reasonable. :) 

### What changes are included in this PR?

Exposes the C++ append parameter from FileOutputStream  to PyArrow's OSFile.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Can add `a` or `ab` to `mode` parameter in `pyarrow.OsFile`

* Closes: #38857

Authored-by: Miles Granger <miles59923@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>

python/pyarrow/includes/libarrow.pxd

@@ -1385,6 +1385,9 @@ cdef extern from "arrow/io/api.h" namespace "arrow::io" nogil:
         @staticmethod
         CResult[shared_ptr[COutputStream]] Open(const c_string& path)
 
+        @staticmethod
+        CResult[shared_ptr[COutputStream]] Open(const c_string& path, c_bool append)
+
         int file_descriptor()
 
     cdef cppclass ReadableFile(CRandomAccessFile):

python/pyarrow/io.pxi

@@ -111,6 +111,7 @@ cdef class NativeFile(_Weakrefable):
         self.is_readable = False
         self.is_writable = False
         self.is_seekable = False
+        self._is_appending = False
 
     def __dealloc__(self):
         if self.own_file:
@@ -139,12 +140,15 @@ cdef class NativeFile(_Weakrefable):
         * rb: binary read
         * wb: binary write
         * rb+: binary read and write
+        * ab: binary append
         """
         # Emulate built-in file modes
         if self.is_readable and self.is_writable:
             return 'rb+'
         elif self.is_readable:
             return 'rb'
+        elif self.is_writable and self._is_appending:
+            return 'ab'
         elif self.is_writable:
             return 'wb'
         else:
@@ -1113,6 +1117,19 @@ cdef class OSFile(NativeFile):
     'rb'
     b'OSFile'
 
+    Open the file to append:
+
+    >>> with pa.OSFile('example_osfile.arrow', mode='ab') as f:
+    ...     f.mode
+    ...     f.write(b' is super!')
+    ...
+    'ab'
+    10
+    >>> with pa.OSFile('example_osfile.arrow') as f:
+    ...     f.read()
+    ...
+    b'OSFile is super!'
+
     Inspect created OSFile:
 
     >>> pa.OSFile('example_osfile.arrow')
@@ -1134,6 +1151,8 @@ cdef class OSFile(NativeFile):
             self._open_readable(c_path, maybe_unbox_memory_pool(memory_pool))
         elif mode in ('w', 'wb'):
             self._open_writable(c_path)
+        elif mode in ('a', 'ab'):
+            self._open_writable(c_path, append=True)
         else:
             raise ValueError('Invalid file mode: {0}'.format(mode))
 
@@ -1146,10 +1165,11 @@ cdef class OSFile(NativeFile):
         self.is_readable = True
         self.set_random_access_file(<shared_ptr[CRandomAccessFile]> handle)
 
-    cdef _open_writable(self, c_string path):
+    cdef _open_writable(self, c_string path, c_bool append=False):
         with nogil:
-            self.output_stream = GetResultValue(FileOutputStream.Open(path))
+            self.output_stream = GetResultValue(FileOutputStream.Open(path, append))
         self.is_writable = True
+        self._is_appending = append
 
     def fileno(self):
         self._assert_open()

python/pyarrow/lib.pxd

@@ -519,6 +519,7 @@ cdef class NativeFile(_Weakrefable):
         bint is_readable
         bint is_writable
         bint is_seekable
+        bint _is_appending
         bint own_file
 
     # By implementing these "virtual" functions (all functions in Cython

python/pyarrow/tests/test_io.py

@@ -1114,6 +1114,13 @@ def test_os_file_writer(tmpdir):
 
     with pytest.raises(IOError):
         f2.read(5)
+    f2.close()
+
+    # Append
+    with pa.OSFile(path, mode='ab') as f4:
+        f4.write(b'bar')
+    with pa.OSFile(path) as f5:
+        assert f5.size() == 6  # foo + bar
 
 
 def test_native_file_write_reject_unicode():
@@ -1152,6 +1159,18 @@ def test_native_file_modes(tmpdir):
         assert f.writable()
         assert not f.seekable()
 
+    with pa.OSFile(path, mode='ab') as f:
+        assert f.mode == 'ab'
+        assert not f.readable()
+        assert f.writable()
+        assert not f.seekable()
+
+    with pa.OSFile(path, mode='a') as f:
+        assert f.mode == 'ab'
+        assert not f.readable()
+        assert f.writable()
+        assert not f.seekable()
+
     with open(path, 'wb') as f:
         f.write(b'foooo')