Refactor to_dataframe deterministicly update progress bar.
#8303
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Previously, a background thread was used to collect progress bar updates from worker threads. So as not to block downloads for progress bar updates, `put_nowait` was used to make progress bar updates. Missed writes to the progress bar were ignored. This caused non-deterministic progress bar updates and test flakiness. Now, worker threads push dataframes to the queue, and the return values for `download_dataframe_bqstorage` and `download_dataframe_tabledata_list` have been updated to return an iterable of pandas DataFrame objects instead of a single DataFrame. This allows progress bar updates to be done independently of which underlying API is used to download the DataFrames. Also, the logic for working with pandas has been moved to the `_pandas_helpers` module.
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| # prevents the queue from filling up, because the main thread | ||
| # has smaller gaps in time between calls to the queue's get | ||
| # method. For a detailed explaination, see: | ||
| # https://friendliness.dev/2019/06/18/python-nowait/ |
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Previously, a background thread was used to collect progress bar updates
from worker threads. So as not to block downloads for progress bar
updates,
put_nowaitwas used to make progress bar updates. Missedwrites to the progress bar were ignored. This caused non-deterministic
progress bar updates and test flakiness.
Now, worker threads push dataframes to the queue, and the return values
for
download_dataframe_bqstorageanddownload_dataframe_tabledata_listhave been updated to return aniterable of pandas DataFrame objects instead of a single DataFrame. This
allows progress bar updates to be done independently of which underlying
API is used to download the DataFrames.
Also, the logic for working with pandas has been moved to the
_pandas_helpersmodule.Closes #8175