Initial guidance on using type annotations

process/Style-guide.md

@@ -18,7 +18,7 @@ variations and expansion spelled out in the sections below.
 
 ## Compatibility
 
-All new log2timeline code should be compatible with Python 3.5+.
+All new log2timeline code should be compatible with Python 3.6+.
 
 ## Indentation
 
@@ -37,7 +37,6 @@ Type | Public | Internal
 ---- | ---- | ----
 Functions | **CapWords()** | **_CapWords()** (protected) and **__CapWords()** (private)
 
-
 ## Linting
 
 log2timeline uses pylint 2.4.x to enforce some additional best practices to keep the source code more readable. These are:
@@ -48,6 +47,28 @@ Other things to keep in mind when using Pylint:
 * Use textual pylint overrides e.g. "# pylint: disable=no-self-argument" instead of "# pylint: disable=E0213". For a list of overrides see: http://docs.pylint.org/features.html
 * The canonical pylint configuration for log2timeline project is generated by l2tdevtools. 
 
+### Type annotations
+
+* Do not use `from __future__ import annotations` since it is not compatible with Python 3.6
+* For importing typing see: http://google.github.io/styleguide/pyguide.html#31912-imports-for-typing
+* Define type annotations as strings so `'int'` instead of `int`. This prevents having issues where types are self referencing
+* Type annotate function or method arguments and return types
+```python
+
+  def _GetNumberOfDaysInYear(self, year: 'int') -> 'int':
+    """Retrieves the number of days in a specific year."""
+```
+
+* Type annotate instance attributes
+```python
+
+ def __init__(self, timestamp: 'Optional[int]' = None) -> 'None':
+    """Initializes a WebKit timestamp."""
+    super(WebKitTime, self).__init__()
+    self._precision: 'str' = definitions.PRECISION_1_MICROSECOND
+    self._timestamp: 'Union[int, None]' = timestamp
+```
+* Limit the usage of `Optional` to function or method arguments, use `Union` in other cases
 
 ### Function and method arguments
 
@@ -56,7 +77,7 @@ Other things to keep in mind when using Pylint:
 ```python
 
 def SomeMethod(self, alternate=False, secondary=True):
-  pass
+  return
 
 # No:
 SomeMethod(True, False)