potiuk commented on pull request #21956:
URL: https://github.com/apache/airflow/pull/21956#issuecomment-1073213869
> I could understand that you are mocking the return of those functions but
I couldn't understand how it's further used in the code. If possible could you
tell me or direct me to some relevant place to learn about it
Sure - this is the key - lambda (in this case) is really an anonymous
function:
This
```
read_from_cache_mock.side_effect = lambda param_name:
cached_values.get(param_name)
```
Is equivalent of this (just written in a shorter way):
```
def read_from_cache_mocked_function(param_name):
return cached_values.get(param_name)
read_from_cache_mock.side_effect = read_from_cache_mocked_function
```
The "side-effect" really points to the function that will be called when the
"read_from_cache_mock" funciton will be actually called. This allows to
implement some logic in your test "mocks" - they become a bit smarter this way.
Usually (in simple cases) your mocks can be told to return specific value:
```
read_from_cache_mock.return_value = x
```
Also you can tell it to return different values if called multiple times by
assigning an iterable to return value:
```
read_from_cache_mock.return_value = [return_when_called_first_time,
return_when_called_second_time, return_when_called_third_time]
```
And `side_effect` is the "smartest" way - you just provide your own
(usually very simple) implementation of a function that can return different
return values - for example based on the parameters that are passed.
In the case above, The "read_from_cache_mock" and
"check_cache_and_write_mock" values are really very simple implementation tha
cache the values in memory and replace the more "file-based" cache.
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