Use pre-defined chunksizes instead of hardcoded ones.

The requests one does currently sit at 10KB, the io buffer one at 8KB.
Both are reasonable values and slightly above our current values.

If those values are adjusted, we will profit as well.

pip/download.py

@@ -38,7 +38,7 @@ from pip.vcs import vcs
 from pip._vendor import requests, six
 from pip._vendor.requests.adapters import BaseAdapter, HTTPAdapter
 from pip._vendor.requests.auth import AuthBase, HTTPBasicAuth
-from pip._vendor.requests.models import Response
+from pip._vendor.requests.models import CONTENT_CHUNK_SIZE, Response
 from pip._vendor.requests.structures import CaseInsensitiveDict
 from pip._vendor.requests.packages import urllib3
 from pip._vendor.cachecontrol import CacheControlAdapter
@@ -588,8 +588,12 @@ def _download_url(resp, link, content_file, hashes):
 
     logger.debug('Downloading from URL %s', link)
 
-    downloaded_chunks = written_chunks(progress_indicator(resp_read(4096),
-                                                          4096))
+    downloaded_chunks = written_chunks(
+        progress_indicator(
+            resp_read(CONTENT_CHUNK_SIZE),
+            CONTENT_CHUNK_SIZE
+        )
+    )
     if hashes:
         hashes.check_against_chunks(downloaded_chunks)
     else:

pip/utils/__init__.py

@@ -3,6 +3,7 @@ from __future__ import absolute_import
 from collections import deque
 import contextlib
 import errno
+import io
 import locale
 # we have a submodule named 'logging' which would shadow this if we used the
 # regular name:
@@ -199,7 +200,7 @@ def file_contents(filename):
         return fp.read().decode('utf-8')
 
 
-def read_chunks(file, size=4096):
+def read_chunks(file, size=io.DEFAULT_BUFFER_SIZE):
     """Yield pieces of data from a file-like object until EOF."""
     while True:
         chunk = file.read(size)