Lazy import pip.network in req_file.py

This notably helps pip freeze which doesn't need the network unless a
URL is given as a requirement file.

Author: ichard26

src/pip/_internal/req/req_file.py

@@ -25,13 +25,12 @@
 from pip._internal.cli import cmdoptions
 from pip._internal.exceptions import InstallationError, RequirementsFileParseError
 from pip._internal.models.search_scope import SearchScope
-from pip._internal.network.session import PipSession
-from pip._internal.network.utils import raise_for_status
 from pip._internal.utils.encoding import auto_decode
 from pip._internal.utils.urls import get_url_scheme
 
 if TYPE_CHECKING:
     from pip._internal.index.package_finder import PackageFinder
+    from pip._internal.network.session import PipSession
 
 __all__ = ["parse_requirements"]
 
@@ -133,7 +132,7 @@ def __init__(
 
 def parse_requirements(
     filename: str,
-    session: PipSession,
+    session: "PipSession",
     finder: Optional["PackageFinder"] = None,
     options: Optional[optparse.Values] = None,
     constraint: bool = False,
@@ -210,7 +209,7 @@ def handle_option_line(
     lineno: int,
     finder: Optional["PackageFinder"] = None,
     options: Optional[optparse.Values] = None,
-    session: Optional[PipSession] = None,
+    session: Optional["PipSession"] = None,
 ) -> None:
     if opts.hashes:
         logger.warning(
@@ -278,7 +277,7 @@ def handle_line(
     line: ParsedLine,
     options: Optional[optparse.Values] = None,
     finder: Optional["PackageFinder"] = None,
-    session: Optional[PipSession] = None,
+    session: Optional["PipSession"] = None,
 ) -> Optional[ParsedRequirement]:
     """Handle a single parsed requirements line; This can result in
     creating/yielding requirements, or updating the finder.
@@ -321,7 +320,7 @@ def handle_line(
 class RequirementsFileParser:
     def __init__(
         self,
-        session: PipSession,
+        session: "PipSession",
         line_parser: LineParser,
     ) -> None:
         self._session = session
@@ -526,7 +525,7 @@ def expand_env_variables(lines_enum: ReqFileLines) -> ReqFileLines:
         yield line_number, line
 
 
-def get_file_content(url: str, session: PipSession) -> Tuple[str, str]:
+def get_file_content(url: str, session: "PipSession") -> Tuple[str, str]:
     """Gets the content of a file; it may be a filename, file: URL, or
     http: URL.  Returns (location, content).  Content is unicode.
     Respects # -*- coding: declarations on the retrieved files.
@@ -538,6 +537,9 @@ def get_file_content(url: str, session: PipSession) -> Tuple[str, str]:
 
     # Pip has special support for file:// URLs (LocalFSAdapter).
     if scheme in ["http", "https", "file"]:
+        # Delay importing heavy network modules until absolutely necessary.
+        from pip._internal.network.utils import raise_for_status
+
         resp = session.get(url)
         raise_for_status(resp)
         return resp.url, resp.text