Python lint: Ruff rules for pylint and code complexity (#525)

pyproject.toml

@@ -102,18 +102,36 @@ executionEnvironments = [
   { root = "examples/servers", reportUnusedFunction = false },
 ]
 
-[tool.ruff.lint]
-select = ["C4", "E", "F", "I", "PERF", "UP"]
-ignore = ["PERF203"]
-
 [tool.ruff]
 line-length = 120
 target-version = "py310"
 extend-exclude = ["README.md"]
 
+[tool.ruff.lint]
+select = [
+    "C4",    # flake8-comprehensions
+    "C90",   # mccabe
+    "E",     # pycodestyle
+    "F",     # pyflakes
+    "I",     # isort
+    "PERF",  # Perflint
+    "PL",    # Pylint
+    "UP",    # pyupgrade
+]
+ignore = ["PERF203", "PLC0415", "PLR0402"]
+mccabe.max-complexity = 24  # Default is 10
+
 [tool.ruff.lint.per-file-ignores]
 "__init__.py" = ["F401"]
 "tests/server/fastmcp/test_func_metadata.py" = ["E501"]
+"tests/shared/test_progress_notifications.py" = ["PLW0603"]
+
+[tool.ruff.lint.pylint]
+allow-magic-value-types = ["bytes", "float", "int", "str"]
+max-args = 23         # Default is 5
+max-branches = 23     # Default is 12
+max-returns = 13      # Default is 6
+max-statements = 102  # Default is 50
 
 [tool.uv.workspace]
 members = ["examples/servers/*", "examples/snippets"]

src/mcp/server/fastmcp/server.py

@@ -182,8 +182,7 @@ class FastMCP(Generic[LifespanResultT]):
                 raise ValueError("Cannot specify both auth_server_provider and token_verifier")
             if not auth_server_provider and not token_verifier:
                 raise ValueError("Must specify either auth_server_provider or token_verifier when auth is enabled")
-        else:
+        elif auth_server_provider or token_verifier:
-            if auth_server_provider or token_verifier:
             raise ValueError("Cannot specify auth_server_provider or token_verifier without auth settings")
 
         self._auth_server_provider = auth_server_provider

src/mcp/server/fastmcp/utilities/func_metadata.py

@@ -139,14 +139,14 @@ class FuncMetadata(BaseModel):
             if field_info.alias:
                 key_to_field_info[field_info.alias] = field_info
 
-        for data_key in data.keys():
+        for data_key, data_value in data.items():
             if data_key not in key_to_field_info:
                 continue
 
             field_info = key_to_field_info[data_key]
-            if isinstance(data[data_key], str) and field_info.annotation is not str:
+            if isinstance(data_value, str) and field_info.annotation is not str:
                 try:
-                    pre_parsed = json.loads(data[data_key])
+                    pre_parsed = json.loads(data_value)
                 except json.JSONDecodeError:
                     continue  # Not JSON - skip
                 if isinstance(pre_parsed, str | int | float):

src/mcp/server/lowlevel/server.py

@@ -85,7 +85,6 @@ import mcp.types as types
 from mcp.server.lowlevel.helper_types import ReadResourceContents
 from mcp.server.models import InitializationOptions
 from mcp.server.session import ServerSession
-from mcp.server.stdio import stdio_server as stdio_server
 from mcp.shared.context import RequestContext
 from mcp.shared.exceptions import McpError
 from mcp.shared.message import ServerMessageMetadata, SessionMessage

src/mcp/server/streamable_http.py

@@ -838,9 +838,8 @@ class StreamableHTTPServerTransport:
                             # If this response is for an existing request stream,
                             # send it there
                             target_request_id = response_id
-                        else:
                         # Extract related_request_id from meta if it exists
-                            if (
+                        elif (
                             session_message.metadata is not None
                             and isinstance(
                                 session_message.metadata,