fix: Replace default file_cache usage with simple memory cache (#4184)
fix: Replace default file_cache usage with simple memory cache (#4184)

* fix: Replace default file_cache usage with simple memory cache

The original main.py creates noisy log messages when the function is run file_cache is unavailable when using oauth2client >= 4.0.0:

```
2020-06-26T19:34:03.459Z datastore_export file_cache is unavailable when using oauth2client >= 4.0.0 or google-auth E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export Traceback (most recent call last): E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export   File "/env/local/lib/python3.7/site-packages/googleapiclient/discovery_cache/file_cache.py", line 33, in <module> E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export     from oauth2client.contrib.locked_file import LockedFile E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export ModuleNotFoundError: No module named 'oauth2client' E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export  E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export During handling of the above exception, another exception occurred: E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export  E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export Traceback (most recent call last): E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export   File "/env/local/lib/python3.7/site-packages/googleapiclient/discovery_cache/file_cache.py", line 37, in <module> E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export     from oauth2client.locked_file import LockedFile E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export ModuleNotFoundError: No module named 'oauth2client' E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export  E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export During handling of the above exception, another exception occurred: E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export  E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export Traceback (most recent call last): E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export   File "/env/local/lib/python3.7/site-packages/googleapiclient/discovery_cache/__init__.py", line 44, in autodetect E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export     from . import file_cache E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export   File "/env/local/lib/python3.7/site-packages/googleapiclient/discovery_cache/file_cache.py", line 41, in <module> E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export     "file_cache is unavailable when using oauth2client >= 4.0.0 or google-auth" E  datastore_export 
2020-06-26T19:34:03.459Z datastore_export ImportError: file_cache is unavailable when using oauth2client >= 4.0.0 or google-auth E  datastore_export 
```

While they don't interfere with the function's operation, they are disconcerting and not necessary. This implements the workaround suggested in https://github.com/googleapis/google-api-python-client/issues/325 (which creates an in-memory cache object to use in replacement).

* Add explanatory comment

Add a comment explaining the inclusion of MemoryCache instance in the `build` invocation.

* Update main.py

PEP8-mandated two newlines between top-level elements

Co-authored-by: Christopher Wilcox <1a61e7a0041d068722f1c352424109b22f854ce0@google.com>
Co-authored-by: Leah E. Cole <a5082685b0b89abc4f85079ca3a58177707489f8@users.noreply.github.com>
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples