googleapiclient.http: guard when importing ssl

Fix #191 by adding a guard when importing `ssl`, and replace all direct
references to `SSLError` (the sole member of `ssl` being used) with a
shim, `_ssl_SSLError`. It is prefixed with a '_' to caution users
against relying on the name when importing `googleclient.http`.

`httplib2` also takes a similar approach (see `ssl_SSLError` in
`httplib2/__init__.py`).

Author: rctay

googleapiclient/http.py

@@ -38,11 +38,18 @@
 import os
 import random
 import socket
-import ssl
 import sys
 import time
 import uuid
 
+# TODO(issue 221): Remove this conditional import jibbajabba.
+try:
+  import ssl
+except ImportError:
+  _ssl_SSLError = object()
+else:
+  _ssl_SSLError = ssl.SSLError
+
 from email.generator import Generator
 from email.mime.multipart import MIMEMultipart
 from email.mime.nonmultipart import MIMENonMultipart
@@ -146,7 +153,7 @@ def _retry_request(http, num_retries, req_type, sleep, rand, uri, method, *args,
       exception = None
       resp, content = http.request(uri, method, *args, **kwargs)
     # Retry on SSL errors and socket timeout errors.
-    except ssl.SSLError as ssl_error:
+    except _ssl_SSLError as ssl_error:
       exception = ssl_error
     except socket.error as socket_error:
       # errno's contents differ by platform, so we have to match by name.