(Closes #473) ensures that a file containing only comments does not give a syntax error.

src/fparser/common/readfortran.py

@@ -1186,20 +1186,28 @@ def handle_cf2py_start(self, line):
             return newline
         return line
 
-    def handle_inline_comment(self, line, lineno, quotechar=None):
+    def handle_inline_comment(
+        self,
+        line: str,
+        lineno: int,
+        quotechar=None,
+        buffer_comments_to_fifo: bool = True,
+    ):
         """
         Any in-line comment is extracted from the line. If
-        keep_inline_comments==True then the extracted comments are put back
-        into the fifo sequence where they will subsequently be processed as
+        buffer_comments_to_fifo==True (the default) then the extracted comments are
+        put back into the fifo sequence where they will subsequently be processed as
         a comment line.
 
-        :param str line: line of code from which to remove in-line comment
-        :param int lineno: line-no. in orig. file
+        :param line: line of code from which to remove in-line comment
+        :param lineno: line-no. in orig. file
         :param quotechar: String to use as character-string delimiter
         :type quotechar: {None, str}
+        :param buffer_comments_to_fifo: whether or not to put any comments back into
+            the fifo buffer for future processing.
 
         :return: line_with_no_comments, quotechar, had_comment
-        :rtype: 3-tuple of str, str, bool
+        :rtype: Tuple[str, str, bool]
 
         """
         had_comment = False
@@ -1212,8 +1220,13 @@ def handle_inline_comment(self, line, lineno, quotechar=None):
             # There's no comment on this line
             return line, quotechar, had_comment
 
+        if buffer_comments_to_fifo:
+            put_item = self.fifo_item.append
+        else:
+            # We're not putting any Comments into the FIFO buffer.
+            put_item = lambda x: None
+
         idx = line.find("!")
-        put_item = self.fifo_item.append
         if quotechar is None and idx != -1:
             # first try a quick method:
             newline = line[:idx]
@@ -1245,7 +1258,9 @@ def handle_inline_comment(self, line, lineno, quotechar=None):
                 # go to next iteration:
                 newline = "".join(noncomment_items) + commentline[5:]
                 self.f2py_comment_lines.append(lineno)
-                return self.handle_inline_comment(newline, lineno, quotechar)
+                return self.handle_inline_comment(
+                    newline, lineno, quotechar, buffer_comments_to_fifo
+                )
             put_item(self.comment_item(commentline, lineno, lineno))
             had_comment = True
         return "".join(noncomment_items), newquotechar, had_comment
@@ -1627,6 +1642,25 @@ def get_source_item(self):
             # A blank line is represented as an empty comment
             return Comment("", (startlineno, endlineno), self)
 
+    def is_comment_line(self, line: str) -> bool:
+        """
+        Utility that examines the supplied line of code and determines whether this
+        reader instance would identify it as a comment.
+
+        :param line: the line of Fortran to examine.
+
+        :returns: whether or not the supplied line is considered to be a comment.
+
+        """
+        if self._format.is_fixed:
+            return _is_fix_comment(
+                line, self._format.is_strict, self._format.f2py_enabled
+            )
+        new_line, _, had_comment = self.handle_inline_comment(
+            line, 0, buffer_comments_to_fifo=False
+        )
+        return not new_line.strip() and had_comment
+
 
 class FortranFileReader(FortranReaderBase):
     """

src/fparser/common/tests/test_readfortran.py

@@ -263,6 +263,25 @@ def test_base_handle_multilines(log):
     assert result == expected
 
 
+def test_fortranreaderbase_is_comment_line():
+    """
+    Tests for the is_comment_line() utility method.
+    """
+    reader = FortranStringReader("    ")
+    # Make the reader free-format.
+    reader.set_format(FortranFormat(True, True))
+    assert not reader.is_comment_line("      ")
+    assert reader.is_comment_line("!")
+    assert reader.is_comment_line("!   ")
+    assert not reader.is_comment_line("call a_func()")
+    # Make the reader fixed-format.
+    reader.set_format(FortranFormat(False, True))
+    assert not reader.is_comment_line("      ")
+    assert reader.is_comment_line("!     a comment")
+    assert reader.is_comment_line("c     a comment")
+    assert reader.is_comment_line("c         ")
+
+
 def test_base_fixed_nonlabel(log):
     """
     Tests that FortranReaderBase.get_source_item() logs the correct messages

src/fparser/two/Fortran2003.py

@@ -315,7 +315,9 @@ def match(reader):
         try:
             while True:
                 obj = Program_Unit(reader)
-                content.append(obj)
+                if obj:
+                    # obj could be None if there are only Comments
+                    content.append(obj)
                 add_comments_includes_directives(content, reader)
                 # cause a StopIteration exception if there are no more lines
                 next_line = reader.next()

src/fparser/two/tests/fortran2003/test_program_r201.py

@@ -262,6 +262,16 @@ def test_missing_prog(f2003_create):
     )
     ast = Program(reader)
     assert "END" in str(ast)
+    reader = get_reader(
+        """\
+      ! This is a program without a program declaration
+      end
+      """,
+        isfree=True,
+        ignore_comments=False,
+    )
+    ast = Program(reader)
+    assert "END" in str(ast)
 
 
 @pytest.mark.xfail(reason="Only the main program is output")

src/fparser/two/tests/test_utils.py

@@ -141,3 +141,26 @@ def test_endstmtbase_match():
         require_stmt_type=True,
     )
     assert result == ("SUBROUTINE", Fortran2003.Name("sub"))
+
+
+@pytest.mark.parametrize("ignore_comments", [True, False])
+def test_base_all_comments(ignore_comments):
+    """
+    Check that supplying a reader with text that consists purely of comments does
+    not result in a syntax error from the Base class.
+
+    """
+    # Check with free-format
+    reader = get_reader(
+        "! just a comment\n! and another", isfree=True, ignore_comments=ignore_comments
+    )
+    result = utils.Base(reader)
+    assert result is None
+    # Check for fixed format
+    reader = get_reader(
+        "c     just a comment\n!     and another",
+        isfree=False,
+        ignore_comments=ignore_comments,
+    )
+    result = utils.Base(reader)
+    assert result is None

src/fparser/two/utils.py

@@ -485,20 +485,13 @@ def __new__(cls, string, parent_cls=None, _deepcopy=False):
             raise AssertionError(repr(result))
         # If we get to here then we've failed to match the current line
         if isinstance(string, FortranReaderBase):
-            content = False
-            for index in range(string.linecount):
-                # Check all lines up to this one for content. We
-                # should be able to only check the current line but
-                # but as the line number returned is not always
-                # correct (due to coding errors) we cannot assume the
-                # line pointed to is the line where the error actually
-                # happened.
-                if string.source_lines[index].strip():
-                    content = True
-                    break
-            if not content:
-                # There are no lines in the input or all lines up to
-                # this one are empty or contain only white space. This
+            freader: FortranReaderBase = string
+            if not freader.source_lines or all(
+                (line.strip() == "" or freader.is_comment_line(line))
+                for line in freader.source_lines
+            ):
+                # There are no lines in the input or all lines up to this one
+                # are empty, comments or contain only white space. This
                 # is typically accepted by fortran compilers so we
                 # follow their lead and do not raise an exception.
                 return
@@ -908,6 +901,8 @@ def tofortran(self, tab="", isfix=None):
         :rtype: str
         """
         mylist = []
+        if not self.content:
+            return ""
         start = self.content[0]
         end = self.content[-1]
         extra_tab = ""