Infer type for partial generic type from assignment (#8036)

Code like this no longer requires a type annotation:

```
a = []
if foo():
    a = [1]
```

Work towards #1055.

Author: JukkaL

mypy/checker.py

@@ -2042,6 +2042,7 @@ def check_assignment(self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type
             self.check_assignment_to_multiple_lvalues(lvalue.items, rvalue, rvalue,
                                                       infer_lvalue_type)
         else:
+            self.try_infer_partial_generic_type_from_assignment(lvalue, rvalue)
             lvalue_type, index_lvalue, inferred = self.check_lvalue(lvalue)
             # If we're assigning to __getattr__ or similar methods, check that the signature is
             # valid.
@@ -2141,6 +2142,37 @@ def check_assignment(self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type
                     rvalue_type = remove_instance_last_known_values(rvalue_type)
                 self.infer_variable_type(inferred, lvalue, rvalue_type, rvalue)
 
+    def try_infer_partial_generic_type_from_assignment(self,
+                                                       lvalue: Lvalue,
+                                                       rvalue: Expression) -> None:
+        """Try to infer a precise type for partial generic type from assignment.
+
+        Example where this happens:
+
+            x = []
+            if foo():
+                x = [1]  # Infer List[int] as type of 'x'
+        """
+        if (isinstance(lvalue, NameExpr)
+                and isinstance(lvalue.node, Var)
+                and isinstance(lvalue.node.type, PartialType)):
+            var = lvalue.node
+            typ = lvalue.node.type
+            if typ.type is None:
+                return
+            partial_types = self.find_partial_types(var)
+            if partial_types is None:
+                return
+            rvalue_type = self.expr_checker.accept(rvalue)
+            rvalue_type = get_proper_type(rvalue_type)
+            if isinstance(rvalue_type, Instance):
+                if rvalue_type.type == typ.type:
+                    var.type = rvalue_type
+                    del partial_types[var]
+            elif isinstance(rvalue_type, AnyType):
+                var.type = fill_typevars_with_any(typ.type)
+                del partial_types[var]
+
     def check_compatibility_all_supers(self, lvalue: RefExpr, lvalue_type: Optional[Type],
                                        rvalue: Expression) -> bool:
         lvalue_node = lvalue.node

mypy/errors.py

@@ -598,14 +598,22 @@ def remove_duplicates(self, errors: List[ErrorTuple]) -> List[ErrorTuple]:
         i = 0
         while i < len(errors):
             dup = False
+            # Use slightly special formatting for member conflicts reporting.
+            conflicts_notes = False
+            j = i - 1
+            while j >= 0 and errors[j][0] == errors[i][0]:
+                if errors[j][4].strip() == 'Got:':
+                    conflicts_notes = True
+                j -= 1
             j = i - 1
             while (j >= 0 and errors[j][0] == errors[i][0] and
                     errors[j][1] == errors[i][1]):
                 if (errors[j][3] == errors[i][3] and
                         # Allow duplicate notes in overload conflicts reporting.
-                        not (errors[i][3] == 'note' and
-                             errors[i][4].strip() in allowed_duplicates
-                             or errors[i][4].strip().startswith('def ')) and
+                        not ((errors[i][3] == 'note' and
+                              errors[i][4].strip() in allowed_duplicates)
+                             or (errors[i][4].strip().startswith('def ') and
+                                 conflicts_notes)) and
                         errors[j][4] == errors[i][4]):  # ignore column
                     dup = True
                     break

test-data/unit/check-inference.test

@@ -1367,34 +1367,29 @@ a = []
 a.append(1)
 a.append('')  # E: Argument 1 to "append" of "list" has incompatible type "str"; expected "int"
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyUsingUpdate]
 a = []
 a.extend([''])
 a.append(0)  # E: Argument 1 to "append" of "list" has incompatible type "int"; expected "str"
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyAndNotAnnotated]
 a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyAndReadBeforeAppend]
 a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 if a: pass
 a.xyz  # E: "List[Any]" has no attribute "xyz"
 a.append('')
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyAndIncompleteTypeInAppend]
 a = [] # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 a.append([])
 a()  # E: "List[Any]" not callable
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyAndMultipleAssignment]
 a, b = [], []
@@ -1403,15 +1398,13 @@ b.append('')
 a() # E: "List[int]" not callable
 b() # E: "List[str]" not callable
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyInFunction]
 def f() -> None:
    a = []
    a.append(1)
    a.append('')  # E: Argument 1 to "append" of "list" has incompatible type "str"; expected "int"
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyAndNotAnnotatedInFunction]
 def f() -> None:
@@ -1422,7 +1415,6 @@ def g() -> None: pass
 a = []
 a.append(1)
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyAndReadBeforeAppendInFunction]
 def f() -> None:
@@ -1431,15 +1423,13 @@ def f() -> None:
     a.xyz  # E: "List[Any]" has no attribute "xyz"
     a.append('')
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyInClassBody]
 class A:
    a = []
    a.append(1)
    a.append('')  # E: Argument 1 to "append" of "list" has incompatible type "str"; expected "int"
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyAndNotAnnotatedInClassBody]
 class A:
@@ -1449,7 +1439,6 @@ class B:
     a = []
     a.append(1)
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyInMethod]
 class A:
@@ -1458,14 +1447,12 @@ class A:
         a.append(1)
         a.append('')  # E: Argument 1 to "append" of "list" has incompatible type "str"; expected "int"
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyAndNotAnnotatedInMethod]
 class A:
     def f(self) -> None:
         a = []  # E: Need type annotation for 'a' (hint: "a: List[<type>] = ...")
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyInMethodViaAttribute]
 class A:
@@ -1475,7 +1462,6 @@ class A:
         self.a.append(1)
         self.a.append('')
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferListInitializedToEmptyInClassBodyAndOverriden]
 from typing import List
@@ -1490,57 +1476,49 @@ class B(A):
     def x(self) -> List[int]:  # E: Signature of "x" incompatible with supertype "A"
         return [123]
 [builtins fixtures/list.pyi]
-[out]
 
 [case testInferSetInitializedToEmpty]
 a = set()
 a.add(1)
 a.add('')  # E: Argument 1 to "add" of "set" has incompatible type "str"; expected "int"
 [builtins fixtures/set.pyi]
-[out]
 
 [case testInferSetInitializedToEmptyUsingDiscard]
 a = set()
 a.discard('')
 a.add(0)  # E: Argument 1 to "add" of "set" has incompatible type "int"; expected "str"
 [builtins fixtures/set.pyi]
-[out]
 
 [case testInferSetInitializedToEmptyUsingUpdate]
 a = set()
 a.update({0})
 a.add('')  # E: Argument 1 to "add" of "set" has incompatible type "str"; expected "int"
 [builtins fixtures/set.pyi]
-[out]
 
 [case testInferDictInitializedToEmpty]
 a = {}
 a[1] = ''
 a() # E: "Dict[int, str]" not callable
 [builtins fixtures/dict.pyi]
-[out]
 
 [case testInferDictInitializedToEmptyUsingUpdate]
 a = {}
 a.update({'': 42})
 a() # E: "Dict[str, int]" not callable
 [builtins fixtures/dict.pyi]
-[out]
 
 [case testInferDictInitializedToEmptyUsingUpdateError]
 a = {}  # E: Need type annotation for 'a' (hint: "a: Dict[<type>, <type>] = ...")
 a.update([1, 2])  # E: Argument 1 to "update" of "dict" has incompatible type "List[int]"; expected "Mapping[Any, Any]"
 a()  # E: "Dict[Any, Any]" not callable
 [builtins fixtures/dict.pyi]
-[out]
 
 [case testInferDictInitializedToEmptyAndIncompleteTypeInUpdate]
 a = {} # E: Need type annotation for 'a' (hint: "a: Dict[<type>, <type>] = ...")
 a[1] = {}
 b = {} # E: Need type annotation for 'b' (hint: "b: Dict[<type>, <type>] = ...")
 b[{}] = 1
 [builtins fixtures/dict.pyi]
-[out]
 
 [case testInferDictInitializedToEmptyAndUpdatedFromMethod]
 map = {}
@@ -1557,20 +1535,42 @@ def add():
 [case testSpecialCaseEmptyListInitialization]
 def f(blocks: Any): # E: Name 'Any' is not defined \
                     # N: Did you forget to import it from "typing"? (Suggestion: "from typing import Any")
-    to_process = [] # E: Need type annotation for 'to_process' (hint: "to_process: List[<type>] = ...")
+    to_process = []
     to_process = list(blocks)
 [builtins fixtures/list.pyi]
-[out]
 
 [case testSpecialCaseEmptyListInitialization2]
 def f(blocks: object):
-    to_process = [] # E: Need type annotation for 'to_process' (hint: "to_process: List[<type>] = ...")
+    to_process = []
     to_process = list(blocks) # E: No overload variant of "list" matches argument type "object" \
                               # N: Possible overload variant: \
                               # N:     def [T] __init__(self, x: Iterable[T]) -> List[T] \
                               # N:     <1 more non-matching overload not shown>
 [builtins fixtures/list.pyi]
-[out]
+
+[case testInferListInitializedToEmptyAndAssigned]
+a = []
+if bool():
+    a = [1]
+reveal_type(a) # N: Revealed type is 'builtins.list[builtins.int*]'
+
+def f():
+    return [1]
+b = []
+if bool():
+    b = f()
+reveal_type(b) # N: Revealed type is 'builtins.list[Any]'
+
+d = {}
+if bool():
+    d = {1: 'x'}
+reveal_type(d) # N: Revealed type is 'builtins.dict[builtins.int*, builtins.str*]'
+
+dd = {} # E: Need type annotation for 'dd' (hint: "dd: Dict[<type>, <type>] = ...")
+if bool():
+    dd = [1] # E: Incompatible types in assignment (expression has type "List[int]", variable has type "Dict[Any, Any]")
+reveal_type(dd) # N: Revealed type is 'builtins.dict[Any, Any]'
+[builtins fixtures/dict.pyi]
 
 
 -- Inferring types of variables first initialized to None (partial types)