Emit export errors to stderr to prevent invalid output (python-poetry…

…#4110)

* Emit export errors to stderr to prevent invalid output

An error in export could emit error messages to stdout, causing the requirements.txt output to stdout to be invalid.

Fixes python-poetry#4109

* Looks at stderr for export tests

poetry/console/commands/export.py

@@ -41,7 +41,7 @@ def handle(self) -> None:
 
  locker = self.poetry.locker
  if not locker.is_locked():
- self.line("<comment>The lock file does not exist. Locking.</comment>")
+ self.line_error("<comment>The lock file does not exist. Locking.</comment>")
  options = []
  if self.io.is_debug():
  options.append(("-vvv", None))
@@ -53,7 +53,7 @@ def handle(self) -> None:
  self.call("lock", " ".join(options))
 
  if not locker.is_fresh():
- self.line(
+ self.line_error(
  "<warning>"
  "Warning: The lock file is not up to date with "
  "the latest changes in pyproject.toml. "

tests/console/commands/test_export.py

@@ -77,12 +77,12 @@ def _export_requirements(tester, poetry):
 def test_export_exports_requirements_txt_file_locks_if_no_lock_file(tester, poetry):
  assert not poetry.locker.lock.exists()
  _export_requirements(tester, poetry)
- assert "The lock file does not exist. Locking." in tester.io.fetch_output()
+ assert "The lock file does not exist. Locking." in tester.io.fetch_error()
 
 
 def test_export_exports_requirements_txt_uses_lock_file(tester, poetry, do_lock):
  _export_requirements(tester, poetry)
- assert "The lock file does not exist. Locking." not in tester.io.fetch_output()
+ assert "The lock file does not exist. Locking." not in tester.io.fetch_error()
 
 
 def test_export_fails_on_invalid_format(tester, do_lock):