Use asserts instead of casts where possible (#14860)

There are many places in mypy's code where `cast`s are currently used
unnecessarily. These can be replaced with `assert`s, which are much more
type-safe, and more mypyc-friendly.

Author: AlexWaygood

mypy/api.py

@@ -47,7 +47,7 @@
 
 import sys
 from io import StringIO
-from typing import Callable, TextIO, cast
+from typing import Callable, TextIO
 
 
 def _run(main_wrapper: Callable[[TextIO, TextIO], None]) -> tuple[str, str, int]:
@@ -59,7 +59,8 @@ def _run(main_wrapper: Callable[[TextIO, TextIO], None]) -> tuple[str, str, int]
         main_wrapper(stdout, stderr)
         exit_status = 0
     except SystemExit as system_exit:
-        exit_status = cast(int, system_exit.code)
+        assert isinstance(system_exit.code, int)
+        exit_status = system_exit.code
 
     return stdout.getvalue(), stderr.getvalue(), exit_status
 

mypy/checker.py

@@ -629,7 +629,8 @@ def _visit_overloaded_func_def(self, defn: OverloadedFuncDef) -> None:
 
         if defn.is_property:
             # HACK: Infer the type of the property.
-            self.visit_decorator(cast(Decorator, defn.items[0]))
+            assert isinstance(defn.items[0], Decorator)
+            self.visit_decorator(defn.items[0])
         for fdef in defn.items:
             assert isinstance(fdef, Decorator)
             self.check_func_item(fdef.func, name=fdef.func.name, allow_empty=True)
@@ -2575,9 +2576,8 @@ def check_import(self, node: ImportBase) -> None:
             if lvalue_type is None:
                 # TODO: This is broken.
                 lvalue_type = AnyType(TypeOfAny.special_form)
-            message = message_registry.INCOMPATIBLE_IMPORT_OF.format(
-                cast(NameExpr, assign.rvalue).name
-            )
+            assert isinstance(assign.rvalue, NameExpr)
+            message = message_registry.INCOMPATIBLE_IMPORT_OF.format(assign.rvalue.name)
             self.check_simple_assignment(
                 lvalue_type,
                 assign.rvalue,
@@ -3657,8 +3657,8 @@ def check_lvalue(self, lvalue: Lvalue) -> tuple[Type | None, IndexExpr | None, V
             not isinstance(lvalue, NameExpr) or isinstance(lvalue.node, Var)
         ):
             if isinstance(lvalue, NameExpr):
-                inferred = cast(Var, lvalue.node)
-                assert isinstance(inferred, Var)
+                assert isinstance(lvalue.node, Var)
+                inferred = lvalue.node
             else:
                 assert isinstance(lvalue, MemberExpr)
                 self.expr_checker.accept(lvalue.expr)
@@ -4984,7 +4984,8 @@ def intersect_instance_callable(self, typ: Instance, callable_type: CallableType
         # In order for this to work in incremental mode, the type we generate needs to
         # have a valid fullname and a corresponding entry in a symbol table. We generate
         # a unique name inside the symbol table of the current module.
-        cur_module = cast(MypyFile, self.scope.stack[0])
+        cur_module = self.scope.stack[0]
+        assert isinstance(cur_module, MypyFile)
         gen_name = gen_unique_name(f"<callable subtype of {typ.type.name}>", cur_module.names)
 
         # Synthesize a fake TypeInfo
@@ -6196,7 +6197,8 @@ def lookup(self, name: str) -> SymbolTableNode:
         else:
             b = self.globals.get("__builtins__", None)
             if b:
-                table = cast(MypyFile, b.node).names
+                assert isinstance(b.node, MypyFile)
+                table = b.node.names
                 if name in table:
                     return table[name]
             raise KeyError(f"Failed lookup: {name}")
@@ -6210,7 +6212,8 @@ def lookup_qualified(self, name: str) -> SymbolTableNode:
             for i in range(1, len(parts) - 1):
                 sym = n.names.get(parts[i])
                 assert sym is not None, "Internal error: attempted lookup of unknown name"
-                n = cast(MypyFile, sym.node)
+                assert isinstance(sym.node, MypyFile)
+                n = sym.node
             last = parts[-1]
             if last in n.names:
                 return n.names[last]
@@ -6514,7 +6517,8 @@ def is_writable_attribute(self, node: Node) -> bool:
                 return False
             return True
         elif isinstance(node, OverloadedFuncDef) and node.is_property:
-            first_item = cast(Decorator, node.items[0])
+            first_item = node.items[0]
+            assert isinstance(first_item, Decorator)
             return first_item.var.is_settable_property
         return False
 

mypy/checkmember.py

@@ -312,7 +312,8 @@ def analyze_instance_member_access(
 
         if method.is_property:
             assert isinstance(method, OverloadedFuncDef)
-            first_item = cast(Decorator, method.items[0])
+            first_item = method.items[0]
+            assert isinstance(first_item, Decorator)
             return analyze_var(name, first_item.var, typ, info, mx)
         if mx.is_lvalue:
             mx.msg.cant_assign_to_method(mx.context)

mypy/fastparse.py

@@ -665,7 +665,9 @@ def fix_function_overloads(self, stmts: list[Statement]) -> list[Statement]:
                 if current_overload and current_overload_name == last_if_stmt_overload_name:
                     # Remove last stmt (IfStmt) from ret if the overload names matched
                     # Only happens if no executable block had been found in IfStmt
-                    skipped_if_stmts.append(cast(IfStmt, ret.pop()))
+                    popped = ret.pop()
+                    assert isinstance(popped, IfStmt)
+                    skipped_if_stmts.append(popped)
                 if current_overload and skipped_if_stmts:
                     # Add bare IfStmt (without overloads) to ret
                     # Required for mypy to be able to still check conditions

mypy/nodes.py

@@ -2183,7 +2183,8 @@ def name(self) -> str:
 
     def expr(self) -> Expression:
         """Return the expression (the body) of the lambda."""
-        ret = cast(ReturnStmt, self.body.body[-1])
+        ret = self.body.body[-1]
+        assert isinstance(ret, ReturnStmt)
         expr = ret.expr
         assert expr is not None  # lambda can't have empty body
         return expr

mypy/report.py

@@ -12,7 +12,7 @@
 import tokenize
 from abc import ABCMeta, abstractmethod
 from operator import attrgetter
-from typing import Any, Callable, Dict, Iterator, Tuple, cast
+from typing import Any, Callable, Dict, Iterator, Tuple
 from typing_extensions import Final, TypeAlias as _TypeAlias
 from urllib.request import pathname2url
 
@@ -704,8 +704,9 @@ def __init__(self, reports: Reports, output_dir: str) -> None:
         super().__init__(reports, output_dir)
 
         memory_reporter = reports.add_report("memory-xml", "<memory>")
+        assert isinstance(memory_reporter, MemoryXmlReporter)
         # The dependency will be called first.
-        self.memory_xml = cast(MemoryXmlReporter, memory_reporter)
+        self.memory_xml = memory_reporter
 
 
 class XmlReporter(AbstractXmlReporter):

mypy/semanal.py

@@ -1314,7 +1314,8 @@ def analyze_property_with_multi_part_definition(self, defn: OverloadedFuncDef) -
         """
         defn.is_property = True
         items = defn.items
-        first_item = cast(Decorator, defn.items[0])
+        first_item = defn.items[0]
+        assert isinstance(first_item, Decorator)
         deleted_items = []
         for i, item in enumerate(items[1:]):
             if isinstance(item, Decorator):
@@ -1357,7 +1358,8 @@ def analyze_function_body(self, defn: FuncItem) -> None:
             # Bind the type variables again to visit the body.
             if defn.type:
                 a = self.type_analyzer()
-                typ = cast(CallableType, defn.type)
+                typ = defn.type
+                assert isinstance(typ, CallableType)
                 a.bind_function_type_variables(typ, defn)
                 for i in range(len(typ.arg_types)):
                     store_argument_type(defn, i, typ, self.named_type)

mypy/server/astmerge.py

@@ -358,7 +358,8 @@ def fixup_and_reset_typeinfo(self, node: TypeInfo) -> TypeInfo:
         if node in self.replacements:
             # The subclass relationships may change, so reset all caches relevant to the
             # old MRO.
-            new = cast(TypeInfo, self.replacements[node])
+            new = self.replacements[node]
+            assert isinstance(new, TypeInfo)
             type_state.reset_all_subtype_caches_for(new)
         return self.fixup(node)
 

mypy/stats.py

@@ -5,7 +5,7 @@
 import os
 from collections import Counter
 from contextlib import contextmanager
-from typing import Iterator, cast
+from typing import Iterator
 from typing_extensions import Final
 
 from mypy import nodes
@@ -154,10 +154,12 @@ def visit_func_def(self, o: FuncDef) -> None:
                     )
                     return
                 for defn in o.expanded:
-                    self.visit_func_def(cast(FuncDef, defn))
+                    assert isinstance(defn, FuncDef)
+                    self.visit_func_def(defn)
             else:
                 if o.type:
-                    sig = cast(CallableType, o.type)
+                    assert isinstance(o.type, CallableType)
+                    sig = o.type
                     arg_types = sig.arg_types
                     if sig.arg_names and sig.arg_names[0] == "self" and not self.inferred:
                         arg_types = arg_types[1:]

mypy/test/testfinegrained.py

@@ -18,7 +18,7 @@
 import re
 import sys
 import unittest
-from typing import Any, cast
+from typing import Any
 
 import pytest
 
@@ -169,7 +169,8 @@ def get_options(self, source: str, testcase: DataDrivenTestCase, build_cache: bo
 
     def run_check(self, server: Server, sources: list[BuildSource]) -> list[str]:
         response = server.check(sources, export_types=True, is_tty=False, terminal_width=-1)
-        out = cast(str, response["out"] or response["err"])
+        out = response["out"] or response["err"]
+        assert isinstance(out, str)
         return out.splitlines()
 
     def build(self, options: Options, sources: list[BuildSource]) -> list[str]: