[AIRFLOW-2525] Fix PostgresHook.copy_expert to work with "COPY FROM"

[sekikn]

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JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-2525
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a JIRA issue.

Description

• Here are some details about my PR, including screenshots of any UI changes:

For now PostgresHook.copy_expert supports
"COPY TO" but not "COPY FROM", because it
opens a file with write mode and doesn't
commit operations. This PR fixes it by
opening a file with read and write mode
and committing operations at last.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Fixed tests.hooks.test_postgres_hook:TestPostgresHook.test_copy_expert to confirm that the open() function is called with read and write mode and operations are committed at last.
In addition, I confirmed that the fixed version worked with both COPY FROM and COPY TO with PostgreSQL 9.5.12.

Commits

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  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

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  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[codecov-io]

Codecov Report

Merging #3421 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3421      +/-   ##
=========================================
+ Coverage    76.6%   76.6%   +<.01%     
=========================================
  Files         203     203              
  Lines       15026   15027       +1     
=========================================
+ Hits        11510   11511       +1     
  Misses       3516    3516

Impacted Files	Coverage Δ
airflow/hooks/postgres_hook.py	92.59% <100%> (+0.28%)	⬆️

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[jgao54]

lgtm thanks!

[sekikn]

Thanks for reviewing and committing @jgao, but really sorry, I've just noticed I had a mistake.
Opening a file with 'w+' mode truncates it first, so COPY FROM breaks the input file and loads no data. My test was incomplete.

I'll send an additional PR soon, so would you take a look? I'll add a new parameter to PostgresHook.copy_expert for specifying file open mode.
It might be possible to decide file opening mode by parsing sql, but I don't think it's a good idea, since naive implementation (using regex, for example) is flaky.