Adding JSON engine

Modeled closely off of csv engine.

The python file cannot be name json.py as this conflicts with the
import of the json module.

engines/__init__.py

@@ -5,7 +5,8 @@
            "postgres",
            "sqlite",
            "msaccess",
-           "csv"
+           "csv",
+           "jsonengine"
            ]
 
 engine_module_list = [

engines/jsonengine.py

@@ -0,0 +1,91 @@
+"""Engine for writing data to a JSON file"""
+
+import os
+import json
+from retriever.lib.models import Engine, no_cleanup
+
+
+class DummyConnection:
+    def cursor(self):
+        pass
+    def commit(self):
+        pass
+    def rollback(self):
+        pass
+    def close(self):
+        pass
+
+class DummyCursor(DummyConnection):
+    pass
+
+
+class engine(Engine):
+    """Engine instance for writing data to a CSV file."""
+    name = "JSON"
+    abbreviation = "json"
+    datatypes = {
+                 "auto": "INTEGER",
+                 "int": "INTEGER",
+                 "bigint": "INTEGER",
+                 "double": "REAL",
+                 "decimal": "REAL",
+                 "char": "TEXT",
+                 "bool": "INTEGER",
+                 }
+    required_opts = [
+                     ("table_name",
+                      "Format of table name",
+                      "{db}_{table}.json"),
+                     ]
+
+    def create_db(self):
+        """Override create_db since there is no database just a JSON file""" 
+        return None
+
+    def create_table(self):
+        """Create the table by creating an empty json"""
+        self.output_file = open(self.table_name(), "w")
+        self.output_file.write('[')
+
+    def execute(self, statement, commit=True):
+        """Write a line to the output file"""
+        self.output_file.write('\n' + statement + ',')
+
+    def format_insert_value(self, value, datatype):
+        """Cleanup values for inserting
+
+        Replace nulls with empty strings and single quotes with double quotes
+
+        """
+        v = Engine.format_insert_value(self, value, datatype)
+        if v == 'null': return ""
+        try:
+            if len(v) > 1 and v[0] == v[-1] == "'":
+                v = '"%s"' % v[1:-1]
+        except:
+            pass
+        return v
+
+    def insert_statement(self, values):
+        if not hasattr(self, 'auto_column_number'):
+            self.auto_column_number = 1
+        offset = 0
+        for i in range(len(self.table.columns)):
+            column = self.table.columns[i]
+            if 'auto' in column[1][0]:
+                values = values[:i+offset] + [self.auto_column_number] + values[i+offset:]
+                self.auto_column_number += 1
+                offset += 1
+
+        datadict = {column[0]: value for column, value in zip(self.table.columns, values)}
+        return json.dumps(datadict)
+
+    def table_exists(self, dbname, tablename):
+        """Check to see if the data file currently exists"""
+        tablename = self.table_name(name=tablename, dbname=dbname)
+        return os.path.exists(tablename)
+
+    def get_connection(self):
+        """Gets the db connection."""
+        self.get_input()
+        return DummyConnection()