Enable strict optional by default (#4971)

As an exception, tests still don't do this since there are
hundreds of tests that would have to be updated. If tests
explicitly define command line options, strict optional will
be on by default -- basically command line options in tests
default to `--no-strict-optional`.

Also rewrote much of the documentation for strict 
optional checking.

Fixes #359.

Author: JukkaL

docs/source/command_line.rst

@@ -21,7 +21,7 @@ flag (or its long form ``--help``)::
               [--warn-unused-ignores] [--warn-unused-configs]
               [--show-error-context] [--no-implicit-optional] [--no-incremental]
               [--quick-and-dirty] [--cache-dir DIR] [--cache-fine-grained]
-              [--skip-version-check] [--strict-optional]
+              [--skip-version-check] [--no-strict-optional]
               [--strict-optional-whitelist [GLOB [GLOB ...]]]
               [--always-true NAME] [--always-false NAME] [--junit-xml JUNIT_XML]
               [--pdb] [--show-traceback] [--stats] [--inferstats]
@@ -298,11 +298,14 @@ Here are some more useful flags:
 - ``--ignore-missing-imports`` suppresses error messages about imports
   that cannot be resolved (see :ref:`follow-imports` for some examples).
 
-- ``--strict-optional`` enables strict checking of ``Optional[...]``
-  types and ``None`` values. Without this option, mypy doesn't
+- ``--no-strict-optional`` disables strict checking of ``Optional[...]``
+  types and ``None`` values. With this option, mypy doesn't
   generally check the use of ``None`` values -- they are valid
-  everywhere. See :ref:`strict_optional` for more about this feature.
-  This flag will become the default in the near future.
+  everywhere. See :ref:`no_strict_optional` for more about this feature.
+
+  **Note:** Strict optional checking was enabled by default starting in
+  mypy 0.600, and in previous versions it had to be explicitly enabled
+  using ``--strict-optional`` (which is still accepted).
 
 - ``--disallow-untyped-defs`` reports an error whenever it encounters
   a function definition without type annotations.

docs/source/common_issues.rst

@@ -51,7 +51,7 @@ flagged as an error.
 
   If you don't know what types to add, you can use ``Any``, but beware:
 
-- **One of the values involved has type ``Any``.** Extending the above
+- **One of the values involved has type 'Any'.** Extending the above
   example, if we were to leave out the annotation for ``a``, we'd get
   no error:
 
@@ -85,17 +85,16 @@ flagged as an error.
   clarity about the latter use ``--follow-imports=error``.  You can
   read up about these and other useful flags in :ref:`command-line`.
 
-- **A function annotated as returning a non-optional type returns ``None``
+- **A function annotated as returning a non-optional type returns 'None'
   and mypy doesn't complain**.
 
   .. code-block:: python
 
       def foo() -> str:
           return None  # No error!
 
-  By default, the ``None`` value is considered compatible with everything. See
-  :ref:`optional` for details on strict optional checking, which allows mypy to
-  check ``None`` values precisely, and will soon become default.
+  You may have disabled strict optional checking (see
+  :ref:`no_strict_optional` for more).
 
 .. _silencing_checker:
 
@@ -131,6 +130,17 @@ The second line is now fine, since the ignore comment causes the name
     if we did have a stub available for ``frobnicate`` then mypy would
     ignore the ``# type: ignore`` comment and typecheck the stub as usual.
 
+
+Unexpected errors about 'None' and/or 'Optional' types
+------------------------------------------------------
+
+Starting from mypy 0.600, mypy uses
+:ref:`strict optional checking <strict_optional>` by default,
+and ``None`` is not compatible with non-optional types.  It's
+easy to switch back to the older behavior where ``None`` was
+compatible with arbitrary types (see :ref:`no_strict_optional`).
+
+
 Types of empty collections
 --------------------------
 

docs/source/config_file.rst

@@ -92,9 +92,6 @@ The following global flags may only be set in the global section
   per-module sections in the config file that didn't match any
   files processed in the current run.
 
-- ``strict_optional`` (Boolean, default False) enables experimental
-  strict Optional checks.
-
 - ``scripts_are_modules`` (Boolean, default False) makes script ``x``
   become module ``x`` instead of ``__main__``.  This is useful when
   checking multiple scripts in a single run.
@@ -180,6 +177,13 @@ overridden by the pattern sections matching the module name.
 - ``almost_silent`` (Boolean, deprecated) equivalent to
   ``follow_imports=skip``.
 
+- ``strict_optional`` (Boolean, default True) enables or disables
+  strict Optional checks. If False, mypy treats ``None`` as
+  compatible with every type.
+
+  **Note::** This was False by default
+  in mypy versions earlier than 0.600.
+
 - ``disallow_any_unimported`` (Boolean, default false) disallows usage of types that come
   from unfollowed imports (such types become aliases for ``Any``).