Improve heuristic to warn about passing lexer/formatter class (#2123)

Don't rely on the error message since 'missing 1 required positional
argument' can give false positives.  Instead, use issubclass().

1 files changed, 22 insertions, 0 deletions

diff --git a/tests/test_basic_api.py b/tests/test_basic_api.py
index 1767572f..62ea4895 100644
--- a/tests/test_basic_api.py
+++ b/tests/test_basic_api.py
@@ -15,6 +15,7 @@ import pytest
 from pygments import lexers, formatters, lex, format
 from pygments.token import _TokenType, Text
 from pygments.lexer import RegexLexer
+from pygments.formatter import Formatter
 from pygments.formatters.img import FontNotFound
 from pygments.util import ClassNotFound
 
@@ -250,6 +251,27 @@ def test_bare_class_handler():
     else:
         assert False, 'nothing raised'
 
+    # These cases should not trigger this heuristic.
+    class BuggyLexer(RegexLexer):
+        def get_tokens(self, text, extra_argument):
+            pass
+        tokens = {'root': []}
+    try:
+        list(lex('dummy', BuggyLexer()))
+    except TypeError as e:
+        assert 'lex() argument must be a lexer instance' not in str(e)
+    else:
+        assert False, 'no error raised by buggy lexer?'
+
+    class BuggyFormatter(Formatter):
+        def format(self, tokensource, outfile, extra_argument):
+            pass
+    try:
+        format([], BuggyFormatter())
+    except TypeError as e:
+        assert 'format() argument must be a formatter instance' not in str(e)
+    else:
+        assert False, 'no error raised by buggy formatter?'
 
 class TestFilters: