Fail if a provided suffix cannot be applied

If a user has provided an output filename for a document, then we should
no longer accept suffixes. The reason is that we can't do something
meaningful with it, as we can't alter the provided output filename.

The proper behavior is to reject this action with an exception. Note
that this acts more of a safeguard, since (currently) there is no path
where a user may add a suffix to a document that already has an output
filename.

dangerzone/document.py

@@ -109,7 +109,10 @@ class Document:
 
     @suffix.setter
     def suffix(self, suf: str) -> None:
-        self._suffix = suf
+        if self._output_filename is None:
+            self._suffix = suf
+        else:
+            raise errors.SuffixNotApplicableException()
 
     @property
     def default_output_filename(self) -> str:

dangerzone/errors.py

@@ -71,6 +71,13 @@ class OutputDirIsNotDirException(DocumentFilenameException):
         super().__init__("Specified output directory is actually not a directory")
 
 
+class SuffixNotApplicableException(DocumentFilenameException):
+    """Exception for when the suffix cannot be applied to the output filename."""
+
+    def __init__(self) -> None:
+        super().__init__("Cannot set a suffix after setting an output filename")
+
+
 def handle_document_errors(func: F) -> F:
     """Log document-related errors and exit gracefully."""
 

tests/test_document.py

@@ -113,6 +113,10 @@ def test_set_output_filename_suffix(sample_doc: str) -> None:
     d.suffix = safe_extension
     assert d.output_filename.endswith(safe_extension)
 
+    d.output_filename = "something_else.pdf"
+    with pytest.raises(errors.SuffixNotApplicableException) as e:
+        d.suffix = "-new-trusted.pdf"
+
 
 def test_is_unconverted_by_default(sample_doc: str) -> None:
     d = Document(sample_doc)