mypy plugin: More precisely detect when fields are required. (#4086)

The mypy plugin would previously incorrectly determine that a field was
not required in a few scenarios where the field really is required. This
specifically affected cases when the `Field()` function is used, where
the plugin assumed that the first argument would always be `default`.

This changes the code to examine each argument more closely, and it now
properly handles several more scenarios where the default is explicitly
named or when the default_factory named argument is used.

changes/4086-richardxia.md

@@ -0,0 +1 @@
+Improve mypy plugin's ability to detect required fields.

pydantic/mypy.py

@@ -494,8 +494,15 @@ def get_is_required(cls: ClassDef, stmt: AssignmentStmt, lhs: NameExpr) -> bool:
             return True
         if isinstance(expr, CallExpr) and isinstance(expr.callee, RefExpr) and expr.callee.fullname == FIELD_FULLNAME:
             # The "default value" is a call to `Field`; at this point, the field is
-            # only required if default is Ellipsis (i.e., `field_name: Annotation = Field(...)`)
-            return len(expr.args) > 0 and expr.args[0].__class__ is EllipsisExpr
+            # only required if default is Ellipsis (i.e., `field_name: Annotation = Field(...)`) or if default_factory
+            # is specified.
+            for arg, name in zip(expr.args, expr.arg_names):
+                # If name is None, then this arg is the default because it is the only positonal argument.
+                if name is None or name == 'default':
+                    return arg.__class__ is EllipsisExpr
+                if name == 'default_factory':
+                    return False
+            return True
         # Only required if the "default value" is Ellipsis (i.e., `field_name: Annotation = ...`)
         return isinstance(expr, EllipsisExpr)
 

tests/mypy/modules/fail_defaults.py

@@ -0,0 +1,17 @@
+from pydantic import BaseModel, Field
+
+
+class Model(BaseModel):
+    # Required
+    undefined_default_no_args: int = Field()
+    undefined_default: int = Field(description='my desc')
+    positional_ellipsis_default: int = Field(...)
+    named_ellipsis_default: int = Field(default=...)
+
+    # Not required
+    positional_default: int = Field(1)
+    named_default: int = Field(default=2)
+    named_default_factory: int = Field(default_factory=lambda: 3)
+
+
+Model()

tests/mypy/outputs/fail_defaults.txt

@@ -0,0 +1,4 @@
+17: error: Missing named argument "undefined_default_no_args" for "Model"  [call-arg]
+17: error: Missing named argument "undefined_default" for "Model"  [call-arg]
+17: error: Missing named argument "positional_ellipsis_default" for "Model"  [call-arg]
+17: error: Missing named argument "named_ellipsis_default" for "Model"  [call-arg]

tests/mypy/test_mypy.py

@@ -25,6 +25,7 @@
     ('mypy-plugin.ini', 'plugin_fail.py', 'plugin-fail.txt'),
     ('mypy-plugin-strict.ini', 'plugin_success.py', 'plugin-success-strict.txt'),
     ('mypy-plugin-strict.ini', 'plugin_fail.py', 'plugin-fail-strict.txt'),
+    ('mypy-plugin-strict.ini', 'fail_defaults.py', 'fail_defaults.txt'),
     ('mypy-default.ini', 'success.py', None),
     ('mypy-default.ini', 'fail1.py', 'fail1.txt'),
     ('mypy-default.ini', 'fail2.py', 'fail2.txt'),
@@ -40,6 +41,7 @@
     ('pyproject-plugin.toml', 'plugin_fail.py', 'plugin-fail.txt'),
     ('pyproject-plugin-strict.toml', 'plugin_success.py', 'plugin-success-strict.txt'),
     ('pyproject-plugin-strict.toml', 'plugin_fail.py', 'plugin-fail-strict.txt'),
+    ('pyproject-plugin-strict.toml', 'fail_defaults.py', 'fail_defaults.txt'),
 ]
 executable_modules = list({fname[:-3] for _, fname, out_fname in cases if out_fname is None})