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Bump furo from 2022.2.14.1 to 2022.3.4 in /docs (#2906)
[etc/vim.git] / src / black / __init__.py
index 90aad220a9cade11cde10c46ee1688e1f797ece9..c4ec99b441f86f063a660e53f03f7a08df40a685 100644 (file)
@@ -1,4 +1,6 @@
 import asyncio
+from json.decoder import JSONDecodeError
+import json
 from concurrent.futures import Executor, ThreadPoolExecutor, ProcessPoolExecutor
 from contextlib import contextmanager
 from datetime import datetime
@@ -7,7 +9,8 @@ import io
 from multiprocessing import Manager, freeze_support
 import os
 from pathlib import Path
-import regex as re
+from pathspec.patterns.gitwildmatch import GitWildMatchPatternError
+import re
 import signal
 import sys
 import tokenize
@@ -18,34 +21,53 @@ from typing import (
     Generator,
     Iterator,
     List,
+    MutableMapping,
     Optional,
     Pattern,
+    Sequence,
     Set,
     Sized,
     Tuple,
     Union,
 )
 
-from dataclasses import replace
 import click
+from click.core import ParameterSource
+from dataclasses import replace
+from mypy_extensions import mypyc_attr
 
 from black.const import DEFAULT_LINE_LENGTH, DEFAULT_INCLUDES, DEFAULT_EXCLUDES
 from black.const import STDIN_PLACEHOLDER
 from black.nodes import STARS, syms, is_simple_decorator_expression
+from black.nodes import is_string_token
 from black.lines import Line, EmptyLineTracker
 from black.linegen import transform_line, LineGenerator, LN
 from black.comments import normalize_fmt_off
-from black.mode import Mode, TargetVersion
+from black.mode import FUTURE_FLAG_TO_FEATURE, Mode, TargetVersion
 from black.mode import Feature, supports_feature, VERSION_TO_FEATURES
 from black.cache import read_cache, write_cache, get_cache_info, filter_cached, Cache
-from black.concurrency import cancel, shutdown
-from black.output import dump_to_file, diff, color_diff, out, err
-from black.report import Report, Changed
-from black.files import find_project_root, find_pyproject_toml, parse_pyproject_toml
+from black.concurrency import cancel, shutdown, maybe_install_uvloop
+from black.output import dump_to_file, ipynb_diff, diff, color_diff, out, err
+from black.report import Report, Changed, NothingChanged
+from black.files import (
+    find_project_root,
+    find_pyproject_toml,
+    parse_pyproject_toml,
+    find_user_pyproject_toml,
+)
 from black.files import gen_python_files, get_gitignore, normalize_path_maybe_ignore
 from black.files import wrap_stream_for_windows
 from black.parsing import InvalidInput  # noqa F401
 from black.parsing import lib2to3_parse, parse_ast, stringify_ast
+from black.handle_ipynb_magics import (
+    mask_cell,
+    unmask_cell,
+    remove_trailing_semicolon,
+    put_trailing_semicolon_back,
+    TRANSFORMED_MAGICS,
+    PYTHON_CELL_MAGICS,
+    jupyter_dependencies_are_installed,
+)
 
 
 # lib2to3 fork
@@ -54,13 +76,7 @@ from blib2to3.pgen2 import token
 
 from _black_version import version as __version__
 
-# If our environment has uvloop installed lets use it
-try:
-    import uvloop
-
-    uvloop.install()
-except ImportError:
-    pass
+COMPILED = Path(__file__).suffix in (".pyd", ".so")
 
 # types
 FileContent = str
@@ -68,10 +84,6 @@ Encoding = str
 NewLine = str
 
 
-class NothingChanged(UserWarning):
-    """Raised when reformatted code is the same as source."""
-
-
 class WriteBack(Enum):
     NO = 0
     YES = 1
@@ -95,6 +107,8 @@ class WriteBack(Enum):
 # Legacy name, left for integrations.
 FileMode = Mode
 
+DEFAULT_WORKERS = os.cpu_count()
+
 
 def read_pyproject_toml(
     ctx: click.Context, param: click.Parameter, value: Optional[str]
@@ -114,7 +128,7 @@ def read_pyproject_toml(
     except (OSError, ValueError) as e:
         raise click.FileError(
             filename=value, hint=f"Error reading configuration file: {e}"
-        )
+        ) from None
 
     if not config:
         return None
@@ -168,14 +182,19 @@ def validate_regex(
     ctx: click.Context,
     param: click.Parameter,
     value: Optional[str],
-) -> Optional[Pattern]:
+) -> Optional[Pattern[str]]:
     try:
         return re_compile_maybe_verbose(value) if value is not None else None
-    except re.error:
-        raise click.BadParameter("Not a valid regular expression")
+    except re.error as e:
+        raise click.BadParameter(f"Not a valid regular expression: {e}") from None
 
 
-@click.command(context_settings=dict(help_option_names=["-h", "--help"]))
+@click.command(
+    context_settings={"help_option_names": ["-h", "--help"]},
+    # While Click does set this field automatically using the docstring, mypyc
+    # (annoyingly) strips 'em so we need to set it here too.
+    help="The uncompromising code formatter.",
+)
 @click.option("-c", "--code", type=str, help="Format the code passed in as a string.")
 @click.option(
     "-l",
@@ -204,6 +223,24 @@ def validate_regex(
         " when piping source on standard input)."
     ),
 )
+@click.option(
+    "--ipynb",
+    is_flag=True,
+    help=(
+        "Format all input files like Jupyter Notebooks regardless of file extension "
+        "(useful when piping source on standard input)."
+    ),
+)
+@click.option(
+    "--python-cell-magics",
+    multiple=True,
+    help=(
+        "When processing Jupyter Notebooks, add the given magic to the list"
+        f" of known python-magics ({', '.join(PYTHON_CELL_MAGICS)})."
+        " Useful for formatting cells with custom python magics."
+    ),
+    default=[],
+)
 @click.option(
     "-S",
     "--skip-string-normalization",
@@ -220,9 +257,14 @@ def validate_regex(
     "--experimental-string-processing",
     is_flag=True,
     hidden=True,
+    help="(DEPRECATED and now included in --preview) Normalize string literals.",
+)
+@click.option(
+    "--preview",
+    is_flag=True,
     help=(
-        "Experimental option that performs more normalization on string literals."
-        " Currently disabled because it leads to some crashes."
+        "Enable potentially disruptive style changes that may be added to Black's main"
+        " functionality in the next major release."
     ),
 )
 @click.option(
@@ -249,6 +291,15 @@ def validate_regex(
     is_flag=True,
     help="If --fast given, skip temporary sanity checks. [default: --safe]",
 )
+@click.option(
+    "--required-version",
+    type=str,
+    help=(
+        "Require a specific version of Black to be running (useful for unifying results"
+        " across many environments e.g. with a pyproject.toml file). It can be"
+        " either a major version number or an exact version."
+    ),
+)
 @click.option(
     "--include",
     type=str,
@@ -302,6 +353,14 @@ def validate_regex(
         "editors that rely on using stdin."
     ),
 )
+@click.option(
+    "-W",
+    "--workers",
+    type=click.IntRange(min=1),
+    default=DEFAULT_WORKERS,
+    show_default=True,
+    help="Number of parallel workers",
+)
 @click.option(
     "-q",
     "--quiet",
@@ -320,7 +379,10 @@ def validate_regex(
         " due to exclusion patterns."
     ),
 )
-@click.version_option(version=__version__)
+@click.version_option(
+    version=__version__,
+    message=f"%(prog)s, %(version)s (compiled: {'yes' if COMPILED else 'no'})",
+)
 @click.argument(
     "src",
     nargs=-1,
@@ -328,6 +390,7 @@ def validate_regex(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     ),
     is_eager=True,
+    metavar="SRC ...",
 )
 @click.option(
     "--config",
@@ -344,7 +407,7 @@ def validate_regex(
     help="Read configuration from FILE path.",
 )
 @click.pass_context
-def main(
+def main(  # noqa: C901
     ctx: click.Context,
     code: Optional[str],
     line_length: int,
@@ -354,20 +417,93 @@ def main(
     color: bool,
     fast: bool,
     pyi: bool,
+    ipynb: bool,
+    python_cell_magics: Sequence[str],
     skip_string_normalization: bool,
     skip_magic_trailing_comma: bool,
     experimental_string_processing: bool,
+    preview: bool,
     quiet: bool,
     verbose: bool,
-    include: Pattern,
-    exclude: Optional[Pattern],
-    extend_exclude: Optional[Pattern],
-    force_exclude: Optional[Pattern],
+    required_version: Optional[str],
+    include: Pattern[str],
+    exclude: Optional[Pattern[str]],
+    extend_exclude: Optional[Pattern[str]],
+    force_exclude: Optional[Pattern[str]],
     stdin_filename: Optional[str],
+    workers: int,
     src: Tuple[str, ...],
     config: Optional[str],
 ) -> None:
     """The uncompromising code formatter."""
+    ctx.ensure_object(dict)
+
+    if src and code is not None:
+        out(
+            main.get_usage(ctx)
+            + "\n\n'SRC' and 'code' cannot be passed simultaneously."
+        )
+        ctx.exit(1)
+    if not src and code is None:
+        out(main.get_usage(ctx) + "\n\nOne of 'SRC' or 'code' is required.")
+        ctx.exit(1)
+
+    root, method = find_project_root(src) if code is None else (None, None)
+    ctx.obj["root"] = root
+
+    if verbose:
+        if root:
+            out(
+                f"Identified `{root}` as project root containing a {method}.",
+                fg="blue",
+            )
+
+            normalized = [
+                (normalize_path_maybe_ignore(Path(source), root), source)
+                for source in src
+            ]
+            srcs_string = ", ".join(
+                [
+                    f'"{_norm}"'
+                    if _norm
+                    else f'\033[31m"{source} (skipping - invalid)"\033[34m'
+                    for _norm, source in normalized
+                ]
+            )
+            out(f"Sources to be formatted: {srcs_string}", fg="blue")
+
+        if config:
+            config_source = ctx.get_parameter_source("config")
+            user_level_config = str(find_user_pyproject_toml())
+            if config == user_level_config:
+                out(
+                    "Using configuration from user-level config at "
+                    f"'{user_level_config}'.",
+                    fg="blue",
+                )
+            elif config_source in (
+                ParameterSource.DEFAULT,
+                ParameterSource.DEFAULT_MAP,
+            ):
+                out("Using configuration from project root.", fg="blue")
+            else:
+                out(f"Using configuration in '{config}'.", fg="blue")
+
+    error_msg = "Oh no! 💥 💔 💥"
+    if (
+        required_version
+        and required_version != __version__
+        and required_version != __version__.split(".")[0]
+    ):
+        err(
+            f"{error_msg} The required version `{required_version}` does not match"
+            f" the running version `{__version__}`!"
+        )
+        ctx.exit(1)
+    if ipynb and pyi:
+        err("Cannot pass both `pyi` and `ipynb` flags!")
+        ctx.exit(1)
+
     write_back = WriteBack.from_configuration(check=check, diff=diff, color=color)
     if target_version:
         versions = set(target_version)
@@ -378,53 +514,74 @@ def main(
         target_versions=versions,
         line_length=line_length,
         is_pyi=pyi,
+        is_ipynb=ipynb,
         string_normalization=not skip_string_normalization,
         magic_trailing_comma=not skip_magic_trailing_comma,
         experimental_string_processing=experimental_string_processing,
+        preview=preview,
+        python_cell_magics=set(python_cell_magics),
     )
-    if config and verbose:
-        out(f"Using configuration from {config}.", bold=False, fg="blue")
+
     if code is not None:
-        print(format_str(code, mode=mode))
-        ctx.exit(0)
-    report = Report(check=check, diff=diff, quiet=quiet, verbose=verbose)
-    sources = get_sources(
-        ctx=ctx,
-        src=src,
-        quiet=quiet,
-        verbose=verbose,
-        include=include,
-        exclude=exclude,
-        extend_exclude=extend_exclude,
-        force_exclude=force_exclude,
-        report=report,
-        stdin_filename=stdin_filename,
-    )
+        # Run in quiet mode by default with -c; the extra output isn't useful.
+        # You can still pass -v to get verbose output.
+        quiet = True
 
-    path_empty(
-        sources,
-        "No Python files are present to be formatted. Nothing to do 😴",
-        quiet,
-        verbose,
-        ctx,
-    )
+    report = Report(check=check, diff=diff, quiet=quiet, verbose=verbose)
 
-    if len(sources) == 1:
-        reformat_one(
-            src=sources.pop(),
-            fast=fast,
-            write_back=write_back,
-            mode=mode,
-            report=report,
+    if code is not None:
+        reformat_code(
+            content=code, fast=fast, write_back=write_back, mode=mode, report=report
         )
     else:
-        reformat_many(
-            sources=sources, fast=fast, write_back=write_back, mode=mode, report=report
+        try:
+            sources = get_sources(
+                ctx=ctx,
+                src=src,
+                quiet=quiet,
+                verbose=verbose,
+                include=include,
+                exclude=exclude,
+                extend_exclude=extend_exclude,
+                force_exclude=force_exclude,
+                report=report,
+                stdin_filename=stdin_filename,
+            )
+        except GitWildMatchPatternError:
+            ctx.exit(1)
+
+        path_empty(
+            sources,
+            "No Python files are present to be formatted. Nothing to do 😴",
+            quiet,
+            verbose,
+            ctx,
         )
 
+        if len(sources) == 1:
+            reformat_one(
+                src=sources.pop(),
+                fast=fast,
+                write_back=write_back,
+                mode=mode,
+                report=report,
+            )
+        else:
+            reformat_many(
+                sources=sources,
+                fast=fast,
+                write_back=write_back,
+                mode=mode,
+                report=report,
+                workers=workers,
+            )
+
     if verbose or not quiet:
-        out("Oh no! 💥 💔 💥" if report.return_code else "All done! ✨ 🍰 ✨")
-        click.secho(str(report), err=True)
+        if code is None and (verbose or report.change_count or report.failure_count):
+            out()
+        out(error_msg if report.return_code else "All done! ✨ 🍰 ✨")
+        if code is None:
+            click.echo(str(report), err=True)
     ctx.exit(report.return_code)
 
 
@@ -442,14 +599,11 @@ def get_sources(
     stdin_filename: Optional[str],
 ) -> Set[Path]:
     """Compute the set of files to be formatted."""
-
-    root = find_project_root(src)
     sources: Set[Path] = set()
-    path_empty(src, "No Path provided. Nothing to do 😴", quiet, verbose, ctx)
 
     if exclude is None:
         exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES)
-        gitignore = get_gitignore(root)
+        gitignore = get_gitignore(ctx.obj["root"])
     else:
         gitignore = None
 
@@ -462,7 +616,7 @@ def get_sources(
             is_stdin = False
 
         if is_stdin or p.is_file():
-            normalized_path = normalize_path_maybe_ignore(p, root, report)
+            normalized_path = normalize_path_maybe_ignore(p, ctx.obj["root"], report)
             if normalized_path is None:
                 continue
 
@@ -479,18 +633,25 @@ def get_sources(
             if is_stdin:
                 p = Path(f"{STDIN_PLACEHOLDER}{str(p)}")
 
+            if p.suffix == ".ipynb" and not jupyter_dependencies_are_installed(
+                verbose=verbose, quiet=quiet
+            ):
+                continue
+
             sources.add(p)
         elif p.is_dir():
             sources.update(
                 gen_python_files(
                     p.iterdir(),
-                    root,
+                    ctx.obj["root"],
                     include,
                     exclude,
                     extend_exclude,
                     force_exclude,
                     report,
                     gitignore,
+                    verbose=verbose,
+                    quiet=quiet,
                 )
             )
         elif s == "-":
@@ -506,11 +667,36 @@ def path_empty(
     """
     Exit if there is no `src` provided for formatting
     """
-    if not src and (verbose or not quiet):
-        out(msg)
+    if not src:
+        if verbose or not quiet:
+            out(msg)
         ctx.exit(0)
 
 
+def reformat_code(
+    content: str, fast: bool, write_back: WriteBack, mode: Mode, report: Report
+) -> None:
+    """
+    Reformat and print out `content` without spawning child processes.
+    Similar to `reformat_one`, but for string content.
+
+    `fast`, `write_back`, and `mode` options are passed to
+    :func:`format_file_in_place` or :func:`format_stdin_to_stdout`.
+    """
+    path = Path("<string>")
+    try:
+        changed = Changed.NO
+        if format_stdin_to_stdout(
+            content=content, fast=fast, write_back=write_back, mode=mode
+        ):
+            changed = Changed.YES
+        report.done(path, changed)
+    except Exception as exc:
+        if report.verbose:
+            traceback.print_exc()
+        report.failed(path, str(exc))
+
+
 def reformat_one(
     src: Path, fast: bool, write_back: WriteBack, mode: Mode, report: "Report"
 ) -> None:
@@ -535,6 +721,8 @@ def reformat_one(
         if is_stdin:
             if src.suffix == ".pyi":
                 mode = replace(mode, is_pyi=True)
+            elif src.suffix == ".ipynb":
+                mode = replace(mode, is_ipynb=True)
             if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):
                 changed = Changed.YES
         else:
@@ -560,19 +748,28 @@ def reformat_one(
         report.failed(src, str(exc))
 
 
+# diff-shades depends on being to monkeypatch this function to operate. I know it's
+# not ideal, but this shouldn't cause any issues ... hopefully. ~ichard26
+@mypyc_attr(patchable=True)
 def reformat_many(
-    sources: Set[Path], fast: bool, write_back: WriteBack, mode: Mode, report: "Report"
+    sources: Set[Path],
+    fast: bool,
+    write_back: WriteBack,
+    mode: Mode,
+    report: "Report",
+    workers: Optional[int],
 ) -> None:
     """Reformat multiple files using a ProcessPoolExecutor."""
     executor: Executor
     loop = asyncio.get_event_loop()
-    worker_count = os.cpu_count()
+    worker_count = workers if workers is not None else DEFAULT_WORKERS
     if sys.platform == "win32":
         # Work around https://bugs.python.org/issue26903
+        assert worker_count is not None
         worker_count = min(worker_count, 60)
     try:
         executor = ProcessPoolExecutor(max_workers=worker_count)
-    except (ImportError, OSError):
+    except (ImportError, NotImplementedError, OSError):
         # we arrive here if the underlying system does not support multi-processing
         # like in AWS Lambda or Termux, in which case we gracefully fallback to
         # a ThreadPoolExecutor with just a single worker (more workers would not do us
@@ -663,7 +860,10 @@ async def schedule_formatting(
                     sources_to_cache.append(src)
                 report.done(src, changed)
     if cancelled:
-        await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
+        if sys.version_info >= (3, 7):
+            await asyncio.gather(*cancelled, return_exceptions=True)
+        else:
+            await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
     if sources_to_cache:
         write_cache(cache, sources_to_cache, mode)
 
@@ -683,6 +883,8 @@ def format_file_in_place(
     """
     if src.suffix == ".pyi":
         mode = replace(mode, is_pyi=True)
+    elif src.suffix == ".ipynb":
+        mode = replace(mode, is_ipynb=True)
 
     then = datetime.utcfromtimestamp(src.stat().st_mtime)
     with open(src, "rb") as buf:
@@ -691,6 +893,10 @@ def format_file_in_place(
         dst_contents = format_file_contents(src_contents, fast=fast, mode=mode)
     except NothingChanged:
         return False
+    except JSONDecodeError:
+        raise ValueError(
+            f"File '{src}' cannot be parsed as valid Jupyter notebook."
+        ) from None
 
     if write_back == WriteBack.YES:
         with open(src, "w", encoding=encoding, newline=newline) as f:
@@ -699,7 +905,10 @@ def format_file_in_place(
         now = datetime.utcnow()
         src_name = f"{src}\t{then} +0000"
         dst_name = f"{src}\t{now} +0000"
-        diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
+        if mode.is_ipynb:
+            diff_contents = ipynb_diff(src_contents, dst_contents, src_name, dst_name)
+        else:
+            diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
 
         if write_back == WriteBack.COLOR_DIFF:
             diff_contents = color_diff(diff_contents)
@@ -719,16 +928,27 @@ def format_file_in_place(
 
 
 def format_stdin_to_stdout(
-    fast: bool, *, write_back: WriteBack = WriteBack.NO, mode: Mode
+    fast: bool,
+    *,
+    content: Optional[str] = None,
+    write_back: WriteBack = WriteBack.NO,
+    mode: Mode,
 ) -> bool:
     """Format file on stdin. Return True if changed.
 
+    If content is None, it's read from sys.stdin.
+
     If `write_back` is YES, write reformatted code back to stdout. If it is DIFF,
     write a diff to stdout. The `mode` argument is passed to
     :func:`format_file_contents`.
     """
     then = datetime.utcnow()
-    src, encoding, newline = decode_bytes(sys.stdin.buffer.read())
+
+    if content is None:
+        src, encoding, newline = decode_bytes(sys.stdin.buffer.read())
+    else:
+        src, encoding, newline = content, "utf-8", ""
+
     dst = src
     try:
         dst = format_file_contents(src, fast=fast, mode=mode)
@@ -742,6 +962,9 @@ def format_stdin_to_stdout(
             sys.stdout.buffer, encoding=encoding, newline=newline, write_through=True
         )
         if write_back == WriteBack.YES:
+            # Make sure there's a newline after the content
+            if dst and dst[-1] != "\n":
+                dst += "\n"
             f.write(dst)
         elif write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
             now = datetime.utcnow()
@@ -755,6 +978,19 @@ def format_stdin_to_stdout(
         f.detach()
 
 
+def check_stability_and_equivalence(
+    src_contents: str, dst_contents: str, *, mode: Mode
+) -> None:
+    """Perform stability and equivalence checks.
+
+    Raise AssertionError if source and destination contents are not
+    equivalent, or if a second pass of the formatter would format the
+    content differently.
+    """
+    assert_equivalent(src_contents, dst_contents)
+    assert_stable(src_contents, dst_contents, mode=mode)
+
+
 def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:
     """Reformat contents of a file and return new contents.
 
@@ -765,27 +1001,124 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
     if not src_contents.strip():
         raise NothingChanged
 
-    dst_contents = format_str(src_contents, mode=mode)
+    if mode.is_ipynb:
+        dst_contents = format_ipynb_string(src_contents, fast=fast, mode=mode)
+    else:
+        dst_contents = format_str(src_contents, mode=mode)
     if src_contents == dst_contents:
         raise NothingChanged
 
-    if not fast:
-        assert_equivalent(src_contents, dst_contents)
-
-        # Forced second pass to work around optional trailing commas (becoming
-        # forced trailing commas on pass 2) interacting differently with optional
-        # parentheses.  Admittedly ugly.
-        dst_contents_pass2 = format_str(dst_contents, mode=mode)
-        if dst_contents != dst_contents_pass2:
-            dst_contents = dst_contents_pass2
-            assert_equivalent(src_contents, dst_contents, pass_num=2)
-            assert_stable(src_contents, dst_contents, mode=mode)
-        # Note: no need to explicitly call `assert_stable` if `dst_contents` was
-        # the same as `dst_contents_pass2`.
+    if not fast and not mode.is_ipynb:
+        # Jupyter notebooks will already have been checked above.
+        check_stability_and_equivalence(src_contents, dst_contents, mode=mode)
     return dst_contents
 
 
-def format_str(src_contents: str, *, mode: Mode) -> FileContent:
+def validate_cell(src: str, mode: Mode) -> None:
+    """Check that cell does not already contain TransformerManager transformations,
+    or non-Python cell magics, which might cause tokenizer_rt to break because of
+    indentations.
+
+    If a cell contains ``!ls``, then it'll be transformed to
+    ``get_ipython().system('ls')``. However, if the cell originally contained
+    ``get_ipython().system('ls')``, then it would get transformed in the same way:
+
+        >>> TransformerManager().transform_cell("get_ipython().system('ls')")
+        "get_ipython().system('ls')\n"
+        >>> TransformerManager().transform_cell("!ls")
+        "get_ipython().system('ls')\n"
+
+    Due to the impossibility of safely roundtripping in such situations, cells
+    containing transformed magics will be ignored.
+    """
+    if any(transformed_magic in src for transformed_magic in TRANSFORMED_MAGICS):
+        raise NothingChanged
+    if (
+        src[:2] == "%%"
+        and src.split()[0][2:] not in PYTHON_CELL_MAGICS | mode.python_cell_magics
+    ):
+        raise NothingChanged
+
+
+def format_cell(src: str, *, fast: bool, mode: Mode) -> str:
+    """Format code in given cell of Jupyter notebook.
+
+    General idea is:
+
+      - if cell has trailing semicolon, remove it;
+      - if cell has IPython magics, mask them;
+      - format cell;
+      - reinstate IPython magics;
+      - reinstate trailing semicolon (if originally present);
+      - strip trailing newlines.
+
+    Cells with syntax errors will not be processed, as they
+    could potentially be automagics or multi-line magics, which
+    are currently not supported.
+    """
+    validate_cell(src, mode)
+    src_without_trailing_semicolon, has_trailing_semicolon = remove_trailing_semicolon(
+        src
+    )
+    try:
+        masked_src, replacements = mask_cell(src_without_trailing_semicolon)
+    except SyntaxError:
+        raise NothingChanged from None
+    masked_dst = format_str(masked_src, mode=mode)
+    if not fast:
+        check_stability_and_equivalence(masked_src, masked_dst, mode=mode)
+    dst_without_trailing_semicolon = unmask_cell(masked_dst, replacements)
+    dst = put_trailing_semicolon_back(
+        dst_without_trailing_semicolon, has_trailing_semicolon
+    )
+    dst = dst.rstrip("\n")
+    if dst == src:
+        raise NothingChanged from None
+    return dst
+
+
+def validate_metadata(nb: MutableMapping[str, Any]) -> None:
+    """If notebook is marked as non-Python, don't format it.
+
+    All notebook metadata fields are optional, see
+    https://nbformat.readthedocs.io/en/latest/format_description.html. So
+    if a notebook has empty metadata, we will try to parse it anyway.
+    """
+    language = nb.get("metadata", {}).get("language_info", {}).get("name", None)
+    if language is not None and language != "python":
+        raise NothingChanged from None
+
+
+def format_ipynb_string(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:
+    """Format Jupyter notebook.
+
+    Operate cell-by-cell, only on code cells, only for Python notebooks.
+    If the ``.ipynb`` originally had a trailing newline, it'll be preserved.
+    """
+    trailing_newline = src_contents[-1] == "\n"
+    modified = False
+    nb = json.loads(src_contents)
+    validate_metadata(nb)
+    for cell in nb["cells"]:
+        if cell.get("cell_type", None) == "code":
+            try:
+                src = "".join(cell["source"])
+                dst = format_cell(src, fast=fast, mode=mode)
+            except NothingChanged:
+                pass
+            else:
+                cell["source"] = dst.splitlines(keepends=True)
+                modified = True
+    if modified:
+        dst_contents = json.dumps(nb, indent=1, ensure_ascii=False)
+        if trailing_newline:
+            dst_contents = dst_contents + "\n"
+        return dst_contents
+    else:
+        raise NothingChanged
+
+
+def format_str(src_contents: str, *, mode: Mode) -> str:
     """Reformat a string and return new contents.
 
     `mode` determines formatting options, such as how many characters per line are
@@ -815,19 +1148,26 @@ def format_str(src_contents: str, *, mode: Mode) -> FileContent:
         hey
 
     """
+    dst_contents = _format_str_once(src_contents, mode=mode)
+    # Forced second pass to work around optional trailing commas (becoming
+    # forced trailing commas on pass 2) interacting differently with optional
+    # parentheses.  Admittedly ugly.
+    if src_contents != dst_contents:
+        return _format_str_once(dst_contents, mode=mode)
+    return dst_contents
+
+
+def _format_str_once(src_contents: str, *, mode: Mode) -> str:
     src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
     dst_contents = []
     future_imports = get_future_imports(src_node)
     if mode.target_versions:
         versions = mode.target_versions
     else:
-        versions = detect_target_versions(src_node)
+        versions = detect_target_versions(src_node, future_imports=future_imports)
+
     normalize_fmt_off(src_node)
-    lines = LineGenerator(
-        mode=mode,
-        remove_u_prefix="unicode_literals" in future_imports
-        or supports_feature(versions, Feature.UNICODE_LITERALS),
-    )
+    lines = LineGenerator(mode=mode)
     elt = EmptyLineTracker(is_pyi=mode.is_pyi)
     empty_line = Line(mode=mode)
     after = 0
@@ -864,7 +1204,9 @@ def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
         return tiow.read(), encoding, newline
 
 
-def get_features_used(node: Node) -> Set[Feature]:
+def get_features_used(  # noqa: C901
+    node: Node, *, future_imports: Optional[Set[str]] = None
+) -> Set[Feature]:
     """Return a set of (relatively) new Python features used in this file.
 
     Currently looking for:
@@ -874,20 +1216,34 @@ def get_features_used(node: Node) -> Set[Feature]:
     - positional only arguments in function signatures and lambdas;
     - assignment expression;
     - relaxed decorator syntax;
+    - usage of __future__ flags (annotations);
+    - print / exec statements;
     """
     features: Set[Feature] = set()
+    if future_imports:
+        features |= {
+            FUTURE_FLAG_TO_FEATURE[future_import]
+            for future_import in future_imports
+            if future_import in FUTURE_FLAG_TO_FEATURE
+        }
+
     for n in node.pre_order():
-        if n.type == token.STRING:
-            value_head = n.value[:2]  # type: ignore
+        if is_string_token(n):
+            value_head = n.value[:2]
             if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:
                 features.add(Feature.F_STRINGS)
 
         elif n.type == token.NUMBER:
-            if "_" in n.value:  # type: ignore
+            assert isinstance(n, Leaf)
+            if "_" in n.value:
                 features.add(Feature.NUMERIC_UNDERSCORES)
 
         elif n.type == token.SLASH:
-            if n.parent and n.parent.type in {syms.typedargslist, syms.arglist}:
+            if n.parent and n.parent.type in {
+                syms.typedargslist,
+                syms.arglist,
+                syms.varargslist,
+            }:
                 features.add(Feature.POS_ONLY_ARGUMENTS)
 
         elif n.type == token.COLONEQUAL:
@@ -918,12 +1274,29 @@ def get_features_used(node: Node) -> Set[Feature]:
                         if argch.type in STARS:
                             features.add(feature)
 
+        elif (
+            n.type in {syms.return_stmt, syms.yield_expr}
+            and len(n.children) >= 2
+            and n.children[1].type == syms.testlist_star_expr
+            and any(child.type == syms.star_expr for child in n.children[1].children)
+        ):
+            features.add(Feature.UNPACKING_ON_FLOW)
+
+        elif (
+            n.type == syms.annassign
+            and len(n.children) >= 4
+            and n.children[3].type == syms.testlist_star_expr
+        ):
+            features.add(Feature.ANN_ASSIGN_EXTENDED_RHS)
+
     return features
 
 
-def detect_target_versions(node: Node) -> Set[TargetVersion]:
+def detect_target_versions(
+    node: Node, *, future_imports: Optional[Set[str]] = None
+) -> Set[TargetVersion]:
     """Detect the version to target based on the nodes used."""
-    features = get_features_used(node)
+    features = get_features_used(node, future_imports=future_imports)
     return {
         version for version in TargetVersion if features <= VERSION_TO_FEATURES[version]
     }
@@ -979,22 +1352,24 @@ def get_future_imports(node: Node) -> Set[str]:
     return imports
 
 
-def assert_equivalent(src: str, dst: str, *, pass_num: int = 1) -> None:
+def assert_equivalent(src: str, dst: str) -> None:
     """Raise AssertionError if `src` and `dst` aren't equivalent."""
     try:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(
-            "cannot use --safe with this file; failed to parse source file.  AST"
-            f" error message: {exc}"
-        )
+            "cannot use --safe with this file; failed to parse source file AST: "
+            f"{exc}\n"
+            "This could be caused by running Black with an older Python version "
+            "that does not support new syntax used in your source file."
+        ) from exc
 
     try:
         dst_ast = parse_ast(dst)
     except Exception as exc:
         log = dump_to_file("".join(traceback.format_tb(exc.__traceback__)), dst)
         raise AssertionError(
-            f"INTERNAL ERROR: Black produced invalid code on pass {pass_num}: {exc}. "
+            f"INTERNAL ERROR: Black produced invalid code: {exc}. "
             "Please report a bug on https://github.com/psf/black/issues.  "
             f"This invalid output might be helpful: {log}"
         ) from None
@@ -1005,14 +1380,17 @@ def assert_equivalent(src: str, dst: str, *, pass_num: int = 1) -> None:
         log = dump_to_file(diff(src_ast_str, dst_ast_str, "src", "dst"))
         raise AssertionError(
             "INTERNAL ERROR: Black produced code that is not equivalent to the"
-            f" source on pass {pass_num}.  Please report a bug on "
+            " source.  Please report a bug on "
             f"https://github.com/psf/black/issues.  This diff might be helpful: {log}"
         ) from None
 
 
 def assert_stable(src: str, dst: str, mode: Mode) -> None:
     """Raise AssertionError if `dst` reformats differently the second time."""
-    newdst = format_str(dst, mode=mode)
+    # We shouldn't call format_str() here, because that formats the string
+    # twice and may hide a bug where we bounce back and forth between two
+    # versions.
+    newdst = _format_str_once(dst, mode=mode)
     if dst != newdst:
         log = dump_to_file(
             str(mode),
@@ -1048,7 +1426,7 @@ def patch_click() -> None:
     """
     try:
         from click import core
-        from click import _unicodefun  # type: ignore
+        from click import _unicodefun
     except ModuleNotFoundError:
         return
 
@@ -1060,6 +1438,7 @@ def patch_click() -> None:
 
 
 def patched_main() -> None:
+    maybe_install_uvloop()
     freeze_support()
     patch_click()
     main()