feat: Installation progress bar ✨

Installation can be pretty slow so it'd be nice to provide progress
feedback to the user.

This commit adds a new progress renderer designed for installation:

- The progress bar will wait one refresh cycle (1000ms/6 = 170ms) before
  appearing. This avoids unsightly very short flashes.

- The progress bar is transient (i.e. it will disappear once all
  packages have been installed). This choice was made to avoid adding
  more clutter to pip install's output (despite the download progress
  bar being persistent).

- The progress bar won't be used at all if there's only one package to
  install.

news/12712.feature.rst

@@ -0,0 +1 @@
+Display a transient progress bar during package installation.

src/pip/_internal/cli/progress_bars.py

@@ -1,11 +1,12 @@
 import functools
 import sys
-from typing import Callable, Generator, Iterable, Iterator, Optional, Tuple
+from typing import Callable, Generator, Iterable, Iterator, Optional, Tuple, TypeVar
 
 from pip._vendor.rich.progress import (
     BarColumn,
     DownloadColumn,
     FileSizeColumn,
+    MofNCompleteColumn,
     Progress,
     ProgressColumn,
     SpinnerColumn,
@@ -16,12 +17,14 @@
 )
 
 from pip._internal.cli.spinners import RateLimiter
-from pip._internal.utils.logging import get_indentation
+from pip._internal.req.req_install import InstallRequirement
+from pip._internal.utils.logging import get_console, get_indentation
 
-DownloadProgressRenderer = Callable[[Iterable[bytes]], Iterator[bytes]]
+T = TypeVar("T")
+ProgressRenderer = Callable[[Iterable[T]], Iterator[T]]
 
 
-def _rich_progress_bar(
+def _rich_download_progress_bar(
     iterable: Iterable[bytes],
     *,
     bar_type: str,
@@ -57,6 +60,28 @@ def _rich_progress_bar(
             progress.update(task_id, advance=len(chunk))
 
 
+def _rich_install_progress_bar(
+    iterable: Iterable[InstallRequirement], *, total: int
+) -> Iterator[InstallRequirement]:
+    columns = (
+        TextColumn("{task.fields[indent]}"),
+        BarColumn(),
+        MofNCompleteColumn(),
+        TextColumn("{task.description}"),
+    )
+    console = get_console()
+
+    bar = Progress(*columns, refresh_per_second=6, console=console, transient=True)
+    # Hiding the progress bar at initialization forces a refresh cycle to occur
+    # until the bar appears, avoiding very short flashes.
+    task = bar.add_task("", total=total, indent=" " * get_indentation(), visible=False)
+    with bar:
+        for req in iterable:
+            bar.update(task, description=rf"\[{req.name}]", visible=True)
+            yield req
+            bar.advance(task)
+
+
 def _raw_progress_bar(
     iterable: Iterable[bytes],
     *,
@@ -81,14 +106,28 @@ def write_progress(current: int, total: int) -> None:
 
 def get_download_progress_renderer(
     *, bar_type: str, size: Optional[int] = None
-) -> DownloadProgressRenderer:
+) -> ProgressRenderer[bytes]:
     """Get an object that can be used to render the download progress.
 
     Returns a callable, that takes an iterable to "wrap".
     """
     if bar_type == "on":
-        return functools.partial(_rich_progress_bar, bar_type=bar_type, size=size)
+        return functools.partial(
+            _rich_download_progress_bar, bar_type=bar_type, size=size
+        )
     elif bar_type == "raw":
         return functools.partial(_raw_progress_bar, size=size)
     else:
         return iter  # no-op, when passed an iterator
+
+
+def get_install_progress_renderer(
+    *, bar_type: str, total: int
+) -> ProgressRenderer[InstallRequirement]:
+    """Get an object that can be used to render the install progress.
+    Returns a callable, that takes an iterable to "wrap".
+    """
+    if bar_type == "on":
+        return functools.partial(_rich_install_progress_bar, total=total)
+    else:
+        return iter

src/pip/_internal/commands/install.py

@@ -464,6 +464,7 @@ def run(self, options: Values, args: List[str]) -> int:
                 warn_script_location=warn_script_location,
                 use_user_site=options.use_user_site,
                 pycompile=options.compile,
+                progress_bar=options.progress_bar,
             )
 
             lib_locations = get_lib_location_guesses(

src/pip/_internal/req/__init__.py

@@ -3,6 +3,7 @@
 from dataclasses import dataclass
 from typing import Generator, List, Optional, Sequence, Tuple
 
+from pip._internal.cli.progress_bars import get_install_progress_renderer
 from pip._internal.utils.logging import indent_log
 
 from .req_file import parse_requirements
@@ -41,6 +42,7 @@ def install_given_reqs(
     warn_script_location: bool,
     use_user_site: bool,
     pycompile: bool,
+    progress_bar: str,
 ) -> List[InstallationResult]:
     """
     Install everything in the given list.
@@ -57,8 +59,19 @@ def install_given_reqs(
 
     installed = []
 
+    show_progress = logger.isEnabledFor(logging.INFO) and len(to_install) > 1
+
+    items = iter(to_install.values())
+    if show_progress:
+        renderer = get_install_progress_renderer(
+            bar_type=progress_bar, total=len(to_install)
+        )
+        items = renderer(items)
+
     with indent_log():
-        for req_name, requirement in to_install.items():
+        for requirement in items:
+            req_name = requirement.name
+            assert req_name is not None
             if requirement.should_reinstall:
                 logger.info("Attempting uninstall: %s", req_name)
                 with indent_log():