Improve error feedback of importing exceptions

tests/test_exceptions.py

@@ -55,3 +55,16 @@ def test_shell_parser_run(self):
             self.assertTrue(e.is_not_installed())
         else:
             self.assertTrue(False, "Expected ShellError")
+
+    def test_missing_module_python(self):
+        """Make sure not installed modules raises the correct error"""
+        filename = self.get_temp_filename()
+        import sys
+        temp = os
+        sys.modules['os'] = None
+        import textract
+        from textract.exceptions import MissingModuleError
+        with self.assertRaises(MissingModuleError):
+            textract.process(filename)
+        sys.modules['os'] = temp
+        os.remove(filename)

textract/exceptions.py

@@ -7,12 +7,14 @@ class CommandLineError(Exception):
     errors occur on the command line to provide a useful command line
     interface.
     """
+
     def render(self, msg):
         return msg % vars(self)
 
 
 class ExtensionNotSupported(CommandLineError):
     """This error is raised with unsupported extensions"""
+
     def __init__(self, ext):
         self.ext = ext
 
@@ -36,6 +38,7 @@ class MissingFileError(CommandLineError):
     """This error is raised when the file can not be located at the
     specified path.
     """
+
     def __init__(self, filename):
         self.filename = filename
         self.root, self.ext = os.path.splitext(filename)
@@ -51,6 +54,7 @@ class UnknownMethod(CommandLineError):
     """This error is raised when the specified --method on the command
     line is unknown.
     """
+
     def __init__(self, method):
         self.method = method
 
@@ -64,6 +68,7 @@ class ShellError(CommandLineError):
     """This error is raised when a shell.run returns a non-zero exit code
     (meaning the command failed).
     """
+
     def __init__(self, command, exit_code, stdout, stderr):
         self.command = command
         self.exit_code = exit_code
@@ -97,3 +102,20 @@ def __str__(self):
             return self.not_installed_message()
         else:
             return self.failed_message()
+
+
+class MissingModuleError(CommandLineError):
+    """This error is raised when a dependency module is not installed.
+    """
+
+    def __init__(self, import_error):
+        self.import_error = import_error.__str__()
+        self.missing_module = self.import_error.split('No module named ')[1]
+
+    def __str__(self):
+        return self.render((
+            'Module %(missing_module)s is not installed on your system.\n'
+            'Please make sure the appropriate dependencies are installed \n'
+            'before using textract:\n\n'
+            '    http://textract.readthedocs.org/en/latest/installation.html\n'
+        ))

textract/parsers/__init__.py

@@ -6,6 +6,7 @@
 import importlib
 import glob
 import re
+import pkgutil
 
 from .. import exceptions
 
@@ -62,17 +63,24 @@ def process(filename, input_encoding=None, output_encoding=DEFAULT_OUTPUT_ENCODI
     # the _parser extension
     rel_module = ext + _FILENAME_SUFFIX
 
-    # If we can't import the module, the file extension isn't currently
-    # supported
+    # check if we can import the parser module related to the file extension
     try:
-        filetype_module = importlib.import_module(
-            rel_module, 'textract.parsers'
-        )
-    except ImportError:
-        raise exceptions.ExtensionNotSupported(ext)
+        # check if the module exists in the system
+        is_module = pkgutil.find_loader('textract.parsers'+rel_module)
+
+        if is_module is not None:
+            filetype_module = importlib.import_module(
+                rel_module, 'textract.parsers'
+            )
+        else:
+            # If we can't import the module, the file extension isn't currently
+            # supported
+            raise exceptions.ExtensionNotSupported(ext)
+    except ImportError as e:
+        # Raise the exception of the import failure
+        raise exceptions.MissingModuleError(e)
 
     # do the extraction
-
     parser = filetype_module.Parser()
     return parser.process(filename, input_encoding, output_encoding, **kwargs)