Merge pull request #1822 from davidmreed/warn-empty-output-file

piptools/scripts/compile.py

@@ -443,7 +443,18 @@ def cli(
 
  # Proxy with a LocalRequirementsRepository if --upgrade is not specified
  # (= default invocation)
- if not upgrade and os.path.exists(output_file.name):
+ output_file_exists = os.path.exists(output_file.name)
+ if not upgrade and output_file_exists:
+ output_file_is_empty = os.path.getsize(output_file.name) == 0
+ if upgrade_install_reqs and output_file_is_empty:
+ log.warning(
+ f"WARNING: the output file {output_file.name} exists but is empty. "
+ "Pip-tools cannot upgrade only specific packages (using -P/--upgrade-package) "
+ "without an existing pin file to provide constraints. "
+ "This often occurs if you redirect standard output to your output file, "
+ "as any existing content is truncated."
+ )
+
  # Use a temporary repository to ensure outdated(removed) options from
  # existing requirements.txt wouldn't get into the current repository.
  tmp_repository = PyPIRepository(pip_args, cache_dir=cache_dir)

tests/test_cli_compile.py

@@ -858,6 +858,27 @@ def test_upgrade_packages_option_no_existing_file(pip_conf, runner):
  assert out.exit_code == 0
  assert "small-fake-a==0.2" in out.stderr.splitlines()
  assert "small-fake-b==0.3" in out.stderr.splitlines()
+ assert (
+ "WARNING: the output file requirements.txt exists but is empty"
+ not in out.stderr
+ )
+
+
+def test_upgrade_packages_empty_target_file_warning(pip_conf, runner):
+ """
+ piptools warns the user if --upgrade-package/-P is specified and the
+ output file exists, but is empty.
+ """
+ with open("requirements.in", "w") as req_in:
+ req_in.write("small-fake-a==0.2")
+ with open("requirements.txt", "w") as req_txt:
+ req_txt.write("")
+
+ out = runner.invoke(cli, ["--no-annotate", "-P", "small-fake-a"])
+
+ assert out.exit_code == 0
+ assert "small-fake-a==0.2" in out.stderr.splitlines()
+ assert "WARNING: the output file requirements.txt exists but is empty" in out.stderr
 
 
 @pytest.mark.parametrize(