Enable PERF and T20 ruff rules (#3845)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: strawgate

pyproject.toml

@@ -180,6 +180,7 @@ error-on-warning = true
 fixable = ["ALL"]
 ignore = [
     "COM812",
+    "PERF203", # try-except in loop — all existing hits are intentional (retry loops, error skipping)
     "PLR0913", # Too many arguments, MCP Servers have a lot of arguments, OKAY?!
     "SIM102",  # Dont require combining if statements
 ]
@@ -194,12 +195,14 @@ extend-select = [
     "INP",  # flake8-no-pep420: Require __init__.py in namespace packages
     "ISC",  # flake8-implicit-str-concat: Prevent accidental string concatenation
     "LOG",  # flake8-logging: Catches logging module misuse
+    "PERF", # perflint: Performance anti-patterns (unnecessary copies, allocations)
     "PIE",  # flake8-pie: More idiomatic Python code
     "PLE",  # pylint-error: Catches actual errors (invalid operations, syntax issues)
     "RSE",  # flake8-raise: Unnecessary parentheses on raise
     "RUF",  # Ruff-specific: Modern best practices unique to Ruff
     "SIM",  # flake8-simplify: Simplifies verbose code patterns
     "SLOT", # flake8-slots: Enforce __slots__ where applicable
+    "T20",  # flake8-print: Catch accidental print() in library code
     "TID",  # flake8-tidy-imports: Banned imports and relative import enforcement
     "UP",   # pyupgrade: Modernize syntax for newer Python versions
 ]
@@ -211,6 +214,10 @@ known-first-party = ["fastmcp"]
 "__init__.py" = ["F401", "I001", "RUF013"]
 # allow imports not at the top of the file
 "src/fastmcp/__init__.py" = ["E402"]
+# CLI and example code legitimately uses print() for user-facing output
+"src/fastmcp/cli/**.py" = ["T20"]
+"src/fastmcp/client/oauth_callback.py" = ["T20"]
+"src/fastmcp/contrib/**/example.py" = ["T20"]
 "!src/**.py" = [ # Only enforce extended ruff rules for code in src/
     "B",    # flake8-bugbear
     "C4",   # flake8-comprehensions
@@ -221,12 +228,14 @@ known-first-party = ["fastmcp"]
     "INP",  # flake8-no-pep420
     "ISC",  # flake8-implicit-str-concat
     "LOG",  # flake8-logging
+    "PERF", # perflint
     "PIE",  # flake8-pie
     "PLE",  # pylint-error
     "RSE",  # flake8-raise
     "RUF",  # Ruff-specific
     "SIM",  # flake8-simplify
     "SLOT", # flake8-slots
+    "T20",  # flake8-print
     "TID",  # flake8-tidy-imports
 ]
 

src/fastmcp/cli/generate.py

@@ -261,7 +261,7 @@ def _tool_function_source(tool: mcp.types.Tool) -> str:
 
     # Build call arguments, using parsed versions for JSON params
     call_arg_parts = []
-    for prop_name, _ in properties.items():
+    for prop_name in properties:
         safe_name = _to_python_identifier(prop_name)
         if any(pn == prop_name for pn, _ in json_params):
             call_arg_parts.append(f"{prop_name!r}: {safe_name}_parsed")
@@ -313,8 +313,7 @@ def generate_cli_script(
     lines.append("from rich.console import Console")
     lines.append("")
     lines.append("from fastmcp import Client")
-    for imp in sorted(extra_imports):
-        lines.append(imp)
+    lines.extend(sorted(extra_imports))
     lines.append("")
 
     # --- Transport config ---
@@ -506,8 +505,7 @@ async def get_prompt(
             "# ---------------------------------------------------------------------------"
         )
 
-        for tool in tools:
-            lines.append(_tool_function_source(tool))
+        lines.extend(_tool_function_source(tool) for tool in tools)
 
     # --- Entry point ---
     lines.append("")

src/fastmcp/cli/install/goose.py

@@ -47,8 +47,7 @@ def generate_goose_deeplink(
     extension_id = _slugify(name)
 
     params: list[str] = [f"cmd={quote(command, safe='')}"]
-    for arg in args:
-        params.append(f"arg={quote(arg, safe='')}")
+    params.extend(f"arg={quote(arg, safe='')}" for arg in args)
     params.append(f"id={quote(extension_id, safe='')}")
     params.append(f"name={quote(name, safe='')}")
     params.append(f"description={quote(description, safe='')}")

src/fastmcp/client/sampling/handlers/anthropic.py

@@ -216,12 +216,11 @@ def _convert_to_anthropic_messages(
                         # Extract text content from the result
                         result_content: str | list[TextBlockParam] = ""
                         if item.content:
-                            text_blocks: list[TextBlockParam] = []
-                            for sub_item in item.content:
-                                if isinstance(sub_item, TextContent):
-                                    text_blocks.append(
-                                        TextBlockParam(type="text", text=sub_item.text)
-                                    )
+                            text_blocks: list[TextBlockParam] = [
+                                TextBlockParam(type="text", text=sub_item.text)
+                                for sub_item in item.content
+                                if isinstance(sub_item, TextContent)
+                            ]
                             if len(text_blocks) == 1:
                                 result_content = text_blocks[0]["text"]
                             elif text_blocks:
@@ -270,12 +269,11 @@ def _convert_to_anthropic_messages(
             if isinstance(content, ToolResultContent):
                 result_content_str: str | list[TextBlockParam] = ""
                 if content.content:
-                    text_parts: list[TextBlockParam] = []
-                    for item in content.content:
-                        if isinstance(item, TextContent):
-                            text_parts.append(
-                                TextBlockParam(type="text", text=item.text)
-                            )
+                    text_parts: list[TextBlockParam] = [
+                        TextBlockParam(type="text", text=item.text)
+                        for item in content.content
+                        if isinstance(item, TextContent)
+                    ]
                     if len(text_parts) == 1:
                         result_content_str = text_parts[0]["text"]
                     elif text_parts:

src/fastmcp/client/sampling/handlers/google_genai.py

@@ -280,9 +280,9 @@ def _convert_messages_to_google_genai_content(
 
         # Handle list content (tool calls + results)
         if isinstance(content, list):
-            parts: list[Part] = []
-            for item in content:
-                parts.append(_sampling_content_to_google_genai_part(item))
+            parts: list[Part] = [
+                _sampling_content_to_google_genai_part(item) for item in content
+            ]
 
             if message.role == "user":
                 google_messages.append(UserContent(parts=parts))

src/fastmcp/client/sampling/handlers/openai.py

@@ -231,10 +231,11 @@ def _convert_to_openai_messages(
                         # Collect tool results (added after assistant message)
                         content_text = ""
                         if item.content:
-                            result_texts = []
-                            for sub_item in item.content:
-                                if isinstance(sub_item, TextContent):
-                                    result_texts.append(sub_item.text)
+                            result_texts = [
+                                sub_item.text
+                                for sub_item in item.content
+                                if isinstance(sub_item, TextContent)
+                            ]
                             content_text = "\n".join(result_texts)
                         tool_messages.append(
                             ChatCompletionToolMessageParam(

src/fastmcp/server/providers/prefab_synthesis.py

@@ -173,9 +173,11 @@ def _walk_provider(provider: Provider) -> None:
         if isinstance(inner, FastMCPApp):
             sources.append(inner._local)
         for src in sources:
-            for component in src._components.values():
-                if isinstance(component, Tool) and _is_prefab_tool(component):
-                    results.append(component)
+            results.extend(
+                component
+                for component in src._components.values()
+                if isinstance(component, Tool) and _is_prefab_tool(component)
+            )
 
         # Recurse into aggregate children
         from fastmcp.server.providers.aggregate import AggregateProvider

src/fastmcp/server/transforms/prompts_as_tools.py

@@ -104,7 +104,7 @@ async def list_prompts() -> str:
 
             result: list[dict[str, Any]] = []
             for p in prompts:
-                result.append(
+                result.append(  # noqa: PERF401
                     {
                         "name": p.name,
                         "description": p.description,

src/fastmcp/server/transforms/resources_as_tools.py

@@ -110,7 +110,7 @@ async def list_resources() -> str:
             result: list[dict[str, Any]] = []
 
             for r in resources:
-                result.append(
+                result.append(  # noqa: PERF401
                     {
                         "uri": str(r.uri),
                         "name": r.name,
@@ -120,7 +120,7 @@ async def list_resources() -> str:
                 )
 
             for t in templates:
-                result.append(
+                result.append(  # noqa: PERF401
                     {
                         "uri_template": t.uri_template,
                         "name": t.name,

src/fastmcp/utilities/inspect.py

@@ -136,7 +136,7 @@ async def inspect_fastmcp_v2(mcp: FastMCP[Any]) -> FastMCPInfo:
     # Extract detailed prompt information
     prompt_infos = []
     for prompt in prompts_list:
-        prompt_infos.append(
+        prompt_infos.append(  # noqa: PERF401
             PromptInfo(
                 key=prompt.key,
                 name=prompt.name or prompt.key,
@@ -156,7 +156,7 @@ async def inspect_fastmcp_v2(mcp: FastMCP[Any]) -> FastMCPInfo:
     # Extract detailed resource information
     resource_infos = []
     for resource in resources_list:
-        resource_infos.append(
+        resource_infos.append(  # noqa: PERF401
             ResourceInfo(
                 key=resource.key,
                 uri=str(resource.uri),
@@ -178,7 +178,7 @@ async def inspect_fastmcp_v2(mcp: FastMCP[Any]) -> FastMCPInfo:
     # Extract detailed template information
     template_infos = []
     for template in templates_list:
-        template_infos.append(
+        template_infos.append(  # noqa: PERF401
             TemplateInfo(
                 key=template.key,
                 uri_template=template.uri_template,
@@ -258,7 +258,7 @@ async def inspect_fastmcp_v1(mcp: FastMCP1x) -> FastMCPInfo:
         # Extract detailed tool information from MCP Tool objects
         tool_infos = []
         for mcp_tool in mcp_tools:
-            tool_infos.append(
+            tool_infos.append(  # noqa: PERF401
                 ToolInfo(
                     key=mcp_tool.name,
                     name=mcp_tool.name,
@@ -301,7 +301,7 @@ async def inspect_fastmcp_v1(mcp: FastMCP1x) -> FastMCPInfo:
         # Extract detailed resource information from MCP Resource objects
         resource_infos = []
         for mcp_resource in mcp_resources:
-            resource_infos.append(
+            resource_infos.append(  # noqa: PERF401
                 ResourceInfo(
                     key=str(mcp_resource.uri),
                     uri=str(mcp_resource.uri),
@@ -321,7 +321,7 @@ async def inspect_fastmcp_v1(mcp: FastMCP1x) -> FastMCPInfo:
         # Extract detailed template information from MCP ResourceTemplate objects
         template_infos = []
         for mcp_template in mcp_templates:
-            template_infos.append(
+            template_infos.append(  # noqa: PERF401
                 TemplateInfo(
                     key=str(mcp_template.uriTemplate),
                     uri_template=str(mcp_template.uriTemplate),