Allow warn to be a Callable

When warn is set to a Callable, it is called with a ChevronError
instead of writing to stderr

__init__.py

@@ -1,5 +1,5 @@
 from .chevron.main import main, cli_main
 from .chevron.renderer import render
-from .chevron.tokenizer import ChevronError
+from .chevron.error import ChevronError, ChevronKeyError
 
-__all__ = ['main', 'render', 'cli_main', 'ChevronError']
+__all__ = ['main', 'render', 'cli_main', 'ChevronError', 'ChevronKeyError']

chevron/__init__.py

@@ -1,5 +1,5 @@
 from .main import main, cli_main
 from .renderer import render
-from .tokenizer import ChevronError
+from .error import ChevronError, ChevronKeyError
 
-__all__ = ['main', 'render', 'cli_main', 'ChevronError']
+__all__ = ['main', 'render', 'cli_main', 'ChevronError', 'ChevronKeyError']

chevron/error.py

@@ -0,0 +1,5 @@
+class ChevronError(SyntaxError):
+    pass
+
+class ChevronKeyError(ChevronError):
+    pass

chevron/renderer.py

@@ -9,8 +9,10 @@
     from collections import Sequence, Iterator, Callable
 try:
     from .tokenizer import tokenize
+    from .error import ChevronKeyError
 except (ValueError, SystemError):  # python 2
     from tokenizer import tokenize
+    from .error import ChevronKeyError
 
 
 import sys
@@ -92,7 +94,11 @@ def _get_key(key, scopes, warn=False):
     # We couldn't find the key in any of the scopes
 
     if warn:
-        sys.stderr.write("Could not find key '%s'%s" % (key, linesep))
+        warning = "Could not find key '%s'%s" % (key, linesep)
+        if isinstance(warn, Callable):
+            warn(ChevronKeyError(warning))
+        else:
+            sys.stderr.write(warning)
 
     return ''
 
@@ -172,7 +178,10 @@ def render(template='', data={}, partials_path='.', partials_ext='mustache',
 
     scopes        -- The list of scopes that get_key will look through
 
-    warn -- Issue a warning to stderr when a template substitution isn't found in the data
+    warn          -- If True, issues a warning to stderr when a template
+                     substitution isn't found in the data.
+                     If warn is callable, a ChevronError is passed to the
+                     callable
 
 
     Returns:

chevron/tokenizer.py

@@ -2,9 +2,10 @@
 _CURRENT_LINE = 1
 _LAST_TAG_LINE = None
 
-
-class ChevronError(SyntaxError):
-    pass
+try:
+    from .error import ChevronError
+except (ValueError, SystemError):  # python 2
+    from .error import ChevronError
 
 #
 # Helper functions

test_spec.py

@@ -498,7 +498,34 @@ def test_disabled_partials(self):
         self.assertEqual(resultNone, expected)
         self.assertEqual(resultEmpty, expected)
         os.chdir('..')
+
+    # https://github.com/noahmorrison/chevron/issues/93
+    def test_custom_warning(self):
+        def render_return_warnings(args):
+            warnings = []
+            def add_warning(warning):
+                warnings.append(warning)
 
+            args['warn'] = add_warning
+
+            result = chevron.render(**args)
+            return (result, warnings)
+
+
+        args = {
+            'template': 'before, {{> with_missing_key }}, after',
+            'partials_dict': {
+                'with_missing_key': '{{#missing_key}}bloop{{/missing_key}}',
+            },
+        }
+
+        expected = 'before, , after'
+        (result, warnings) = render_return_warnings(args)
+
+        self.assertEqual(result, expected)
+        self.assertEqual(len(warnings), 1)
+        self.assertEqual(type(warnings[0]), chevron.ChevronKeyError)
+        self.assertEqual(warnings[0].msg, "Could not find key 'missing_key'\n")
 
 # Run unit tests from command line
 if __name__ == "__main__":