chadrik commented on a change in pull request #10822: [BEAM-7746] Minor typing
updates / fixes
URL: https://github.com/apache/beam/pull/10822#discussion_r384807421
##########
File path: sdks/python/apache_beam/transforms/external_java.py
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@@ -37,18 +39,19 @@
# Protect against environments where apitools library is not available.
# pylint: disable=wrong-import-order, wrong-import-position
+apiclient = None # type: Optional[types.ModuleType]
Review comment:
> My question is why we need to type apiclient as types.ModuleType at all.
Fair question.
If we don't do this (i.e. as with the original code), we get the following
error:
```
apache_beam/transforms/external_java.py:46: error: Incompatible types in
assignment (expression has type "None", variable has type Module) [assignment]
```
To resolve this, we need to mark `apiclient` as `Optional`
---
why can't we save ourselves some headache and leave out of the
`types.ModuleType` part?
```python
apiclient = None # type: Optional
try:
from apache_beam.runners.dataflow.internal import apiclient
except ImportError:
pass
```
If we do this, `apiclient` will become `Optional[Any]`
---
Why can't we ignore the error?
We can, but then mypy will mark the type as non-Optional, and that would
remove the added protections that mypy provides against accidentally using the
variable when it's `None`.
---
Why can't we just add the type comment on the original `apiclient = None`
line?
```python
try:
from apache_beam.runners.dataflow.internal import apiclient
except ImportError:
apiclient = None # type: Optional[types.ModuleType]
```
With this, we get the following error:
```
apache_beam/transforms/external_java.py:46: error: Name 'apiclient' already
defined (by an import) [no-redef]
```
There is only one opportunity to override/influence the inferred type of a
variable: on the first line where it is defined (think of type variable
definitions like C/C++, but with python scoping rules). However, `apiclient`
is defined via an import rather than an assignment, which forces us to preface
the import with a variable definition.
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