Add test & behavior: Compile an empty txt when the infile is empty an…

…d --no-header is supplied (Fixes #900)

Add !is_empty (yielded) tracking to the writer's _iter_lines method, and yield a blank line (empty string) if (and only if) otherwise nothing at all would be yielded.

piptools/writer.py

@@ -146,10 +146,14 @@ def _iter_lines(
  warn_uninstallable = False
  has_hashes = hashes and any(hash for hash in hashes.values())
 
+ yielded = False
+
  for line in self.write_header():
  yield line
+ yielded = True
  for line in self.write_flags():
  yield line
+ yielded = True
 
  unsafe_requirements = (
  {r for r in results if r.name in UNSAFE_PACKAGES}
@@ -158,24 +162,26 @@ def _iter_lines(
  )
  packages = {r for r in results if r.name not in UNSAFE_PACKAGES}
 
- packages = sorted(packages, key=self._sort_key)
-
- for ireq in packages:
- if has_hashes and not hashes.get(ireq):
- yield MESSAGE_UNHASHED_PACKAGE
- warn_uninstallable = True
- line = self._format_requirement(
- ireq,
- reverse_dependencies,
- primary_packages,
- markers.get(key_from_ireq(ireq)),
- hashes=hashes,
- )
- yield line
+ if packages:
+ packages = sorted(packages, key=self._sort_key)
+ for ireq in packages:
+ if has_hashes and not hashes.get(ireq):
+ yield MESSAGE_UNHASHED_PACKAGE
+ warn_uninstallable = True
+ line = self._format_requirement(
+ ireq,
+ reverse_dependencies,
+ primary_packages,
+ markers.get(key_from_ireq(ireq)),
+ hashes=hashes,
+ )
+ yield line
+ yielded = True
 
  if unsafe_requirements:
  unsafe_requirements = sorted(unsafe_requirements, key=self._sort_key)
  yield ""
+ yielded = True
  if has_hashes and not self.allow_unsafe:
  yield MESSAGE_UNSAFE_PACKAGES_UNPINNED
  warn_uninstallable = True
@@ -195,6 +201,10 @@ def _iter_lines(
  else:
  yield req
 
+ # Yield even when there's no real content, so that blank files are written
+ if not yielded:
+ yield ""
+
  if warn_uninstallable:
  log.warning(MESSAGE_UNINSTALLABLE)
 

tests/test_cli_compile.py

@@ -801,3 +801,29 @@ def test_dry_run_doesnt_touch_output_file(
  # The output file must not be touched
  after_compile_mtime = os.stat("requirements.txt").st_mtime
  assert after_compile_mtime == before_compile_mtime
+
+
+@pytest.mark.parametrize(
+ "empty_input_pkg, prior_output_pkg",
+ [
+ ("", ""),
+ ("", "small-fake-a==0.1\n"),
+ ("# Nothing to see here", ""),
+ ("# Nothing to see here", "small-fake-a==0.1\n"),
+ ],
+)
+def test_empty_input_file_no_header(runner, empty_input_pkg, prior_output_pkg):
+ """
+ Tests pip-compile creates an empty requirements.txt file,
+ given --no-header and empty requirements.in
+ """
+ with open("requirements.in", "w") as req_in:
+ req_in.write(empty_input_pkg) # empty input file
+
+ with open("requirements.txt", "w") as req_txt:
+ req_txt.write(prior_output_pkg)
+
+ runner.invoke(cli, ["--no-header", "requirements.in"])
+
+ with open("requirements.txt", "r") as req_txt:
+ assert req_txt.read().strip() == ""