Merge #18210: test: type hints in Python tests

bd7e530f010d43816bb05d6f1590d1cd36cdaa2c This PR adds initial support for type hints checking in python scripts. (Kiminuo)

Pull request description:

  This PR adds initial support for type hints checking in python scripts.

  Support for type hints was introduced in Python 3.5. Type hints make it easier to read and review code in my opinion. Also an IDE may discover a potential bug sooner. Yet, as PEP 484 says: "It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention."

  [Mypy](https://mypy.readthedocs.io/en/latest/index.html) is used in `lint-python.sh` to do the type checking. The package is standard so there is little chance that it will be abandoned. Mypy checks that type hints in source code are correct when they are not, it fails with an error.

  **Notes:**

  * [--ignore-missing-imports](https://mypy.readthedocs.io/en/latest/command_line.html#cmdoption-mypy-ignore-missing-imports) switch is passed on to `mypy` checker for now. The effect of this is that one does not need `# type: ignore` for `import zmq`. More information about import processing can be found [here](https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports). This can be changed in a follow-up PR, if it is deemed useful.
  * We are stuck with Python 3.5 until 04/2021 (see https://packages.ubuntu.com/xenial/python3). When Python version is bumped to 3.6+, one can change:
      ```python
      _opcode_instances = []  # type: List[CScriptOp]
      ```
      to
      ```python
      _opcode_instances:List[CScriptOp] = []
      ```
      for type hints that are **not** function parameters and function return types.

  **Useful resources:**

  * https://docs.python.org/3.5/library/typing.html
  * https://www.python.org/dev/peps/pep-0484/

ACKs for top commit:
  fanquake:
    ACK bd7e530f010d43816bb05d6f1590d1cd36cdaa2c - the type checking is not the most robust (there are things it fails to detect), but I think this is worth adopting (in a limited capacity while we maintain 3.5 compat).
  MarcoFalke:
    ACK bd7e530f010d43816bb05d6f1590d1cd36cdaa2c fine with me

Tree-SHA512: 21ef213915fb1dec6012f59ef17484e6c9e0abf542a316b63d5f21a7778ad5ebabf8961ef5fc8e5414726c2ee9c6ae07c7353fb4dd337f8fcef5791199c8987a

1 files changed, 4 insertions, 2 deletions

diff --git a/test/functional/README.md b/test/functional/README.md
index 004e0afb1d..aff5f714f2 100644
--- a/test/functional/README.md
+++ b/test/functional/README.md
@@ -26,10 +26,12 @@ don't have test cases for.
   The Travis linter also checks this, but [possibly not in all cases](https://github.com/bitcoin/bitcoin/pull/14884#discussion_r239585126).
 - See [the python lint script](/test/lint/lint-python.sh) that checks for violations that
   could lead to bugs and issues in the test code.
+- Use [type hints](https://docs.python.org/3/library/typing.html) in your code to improve code readability
+  and to detect possible bugs earlier.
 - Avoid wildcard imports
 - Use a module-level docstring to describe what the test is testing, and how it
   is testing it.
-- When subclassing the BitcoinTestFramwork, place overrides for the
+- When subclassing the BitcoinTestFramework, place overrides for the
   `set_test_params()`, `add_options()` and `setup_xxxx()` methods at the top of
   the subclass, then locally-defined helper methods, then the `run_test()` method.
 - Use `'{}'.format(x)` for string formatting, not `'%s' % x`.
@@ -45,7 +47,7 @@ don't have test cases for.
     - `rpc` for tests for individual RPC methods or features, eg `rpc_listtransactions.py`
     - `tool` for tests for tools, eg `tool_wallet.py`
     - `wallet` for tests for wallet features, eg `wallet_keypool.py`
-- use an underscore to separate words
+- Use an underscore to separate words
     - exception: for tests for specific RPCs or command line options which don't include underscores, name the test after the exact RPC or argument name, eg `rpc_decodescript.py`, not `rpc_decode_script.py`
 - Don't use the redundant word `test` in the name, eg `interface_zmq.py`, not `interface_zmq_test.py`