Complement cheatsheet for Optionals

Add a paragrah to explain there are 2 ways to make the typechecker
understand that an optional is not `None`: use a if condition or an
`assert`.

Author: vincent-prz

docs/source/cheat_sheet.rst

@@ -49,6 +49,15 @@ Built-in types
    if x is not None:
       print x
 
+   # There are 2 ways you can make mypy understand that
+   # an Optional value is not None
+   x = some_function()  # type: Optional[str]
+   # First way: with an if condition
+   if x is not None:
+       print(x.upper())
+   # Second way: with an assert statement
+   assert x is not None
+   print(x.upper())
 
 Functions
 *********

docs/source/cheat_sheet_py3.rst

@@ -75,6 +75,15 @@ Built-in types
    if x is not None:
        print(x)
 
+   # There are 2 ways you can make mypy understand that
+   # an Optional value is not None
+   x: Optional[str] = some_function()
+   # First way: with an if condition
+   if x is not None:
+       print(x.upper())
+   # Second way: with an assert statement
+   assert x is not None
+   print(x.upper())
 
 Functions
 *********