Improve interactive mode detection (theskumar#183)

* Improve interactive mode detection

Previously, this checked whether __file__ was defined in globals().  globals()
is tied to the current module, so this will always be defined. Fix this by
importing __main__ and asking whether it has __file__ defined. This approach is
outlined in stackoverflow.com/a/2356420.

Thanks @andrewsmith

README.md

@@ -299,6 +299,7 @@ Changelog
 Unreleased
 -----
 
+- Improve interactive mode detection ([@andrewsmith])([#183]).
 - Refactor parser to fix parsing inconsistencies ([@bbc2])([#170]).
   - Interpret escapes as control characters only in double-quoted strings.
   - Interpret `#` as start of comment only if preceded by whitespace.
@@ -430,7 +431,9 @@ Unreleased
 [#121]: https://github.com/theskumar/python-dotenv/issues/121
 [#176]: https://github.com/theskumar/python-dotenv/issues/176
 [#170]: https://github.com/theskumar/python-dotenv/issues/170
+[#183]: https://github.com/theskumar/python-dotenv/issues/183
 
+[@andrewsmith]: https://github.com/andrewsmith
 [@asyncee]: https://github.com/asyncee
 [@greyli]: https://github.com/greyli
 [@venthur]: https://github.com/venthur

src/dotenv/main.py

@@ -235,8 +235,14 @@ def find_dotenv(filename='.env', raise_error_if_not_found=False, usecwd=False):
 
     Returns path to the file if found, or an empty string otherwise
     """
-    if usecwd or '__file__' not in globals():
-        # should work without __file__, e.g. in REPL or IPython notebook
+
+    def _is_interactive():
+        """ Decide whether this is running in a REPL or IPython notebook """
+        main = __import__('__main__', None, None, fromlist=['__file__'])
+        return not hasattr(main, '__file__')
+
+    if usecwd or _is_interactive():
+        # Should work without __file__, e.g. in REPL or IPython notebook.
         path = os.getcwd()
     else:
         # will work for .py files