bpo-22908: Add seek and tell functionality to ZipExtFile

Lib/zipfile.py

@@ -772,6 +772,17 @@ def __init__(self, fileobj, mode, zipinfo, decrypter=None,
         else:
             self._expected_crc = None
 
+        self._seekable = False
+        try:
+            if fileobj.seekable():
+                self._orig_compress_start = fileobj.tell()
+                self._orig_compress_size = zipinfo.compress_size
+                self._orig_file_size = zipinfo.file_size
+                self._orig_start_crc = self._running_crc
+                self._seekable = True
+        except AttributeError:
+            pass
+
     def __repr__(self):
         result = ['<%s.%s' % (self.__class__.__module__,
                               self.__class__.__qualname__)]
@@ -957,6 +968,51 @@ def close(self):
         finally:
             super().close()
 
+    def seekable(self):
+        return self._seekable
+
+    def seek(self, offset, from_what = 0):
+        if not self._seekable:
+            raise io.UnsupportedOperation("underlying stream is not seekable")
+        curr_pos = self.tell()
+        new_pos = offset # Default seek from start of file
+        if from_what == 1: # Seek from current position
+            new_pos = curr_pos + offset
+        elif from_what == 2: # Seek from EOF
+            new_pos = self._orig_file_size + offset
+
+        if new_pos > self._orig_file_size:
+            new_pos = self._orig_file_size
+
+        if new_pos < 0:
+            new_pos = 0
+
+        if new_pos > curr_pos:
+            self.read(new_pos - curr_pos)
+        elif new_pos < curr_pos:
+            if self._offset >= curr_pos - new_pos:
+                # No need to reset if the new position is within the read buffer
+                self._offset -= curr_pos - new_pos
+            else:
+                self._fileobj.seek(self._orig_compress_start)
+                self._running_crc = self._orig_start_crc
+                self._compress_left = self._orig_compress_size
+                self._left = self._orig_file_size
+                self._readbuffer = b''
+                self._offset = 0
+                self._decompressor = zipfile._get_decompressor(self._compress_type)
+                self._eof = False
+                if new_pos > 0:
+                    self.read(new_pos)
+
+        return self.tell()
+
+    def tell(self):
+        if not self._seekable:
+            raise io.UnsupportedOperation("underlying stream is not seekable")
+        filepos = self._orig_file_size - self._left - len(self._readbuffer) + self._offset
+        return filepos
+
 
 class _ZipWriteFile(io.BufferedIOBase):
     def __init__(self, zf, zinfo, zip64):

Misc/NEWS.d/next/Library/2017-12-21-22-00-11.bpo-22908.cVm89I.rst

@@ -0,0 +1,4 @@
+Added seek and rest to the ZipExtFile class. This only works if the file
+object used to open the zipfile is seekable. This change is the first
+iteration of this feature and can easily be optimized by considering the
+_readbuffer.