curds: Add a class for pickling and unpickling objects

This gives us a convenient way to handle saved data. We use the 'with'
statment to handle any errors without crashing.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

curds/data.py

@@ -1,12 +1,47 @@
 # Copyright 2019 (c) Anna Schumaker.
 import os
+import pickle
 import xdg.BaseDirectory
 
 __resource = "emmental"
 if os.environ.get("EMMENTAL_TESTING"):
     __resource = "emmental-testing"
 
+READ  = 'rb'
+WRITE = 'wb'
+
 emmental_data = xdg.BaseDirectory.save_data_path(__resource)
 
 def data_file_path(filename):
     return os.path.join(emmental_data, filename)
+
+class DataFile:
+    def __init__(self, path, mode):
+        self.path = data_file_path(path)
+        self.mode = mode
+        self.file = None
+
+    def __enter__(self):
+        path = self.path
+        if self.mode == WRITE:
+            path = data_file_path(f".{os.path.basename(self.path)}.tmp")
+        if self.mode == WRITE or os.path.exists(path):
+            self.file = open(path, self.mode)
+        return self
+
+    def __exit__(self, exp_type, exp_value, traceback):
+        if self.file:
+            self.file.close()
+            self.file = None
+        if self.mode == WRITE:
+            path = data_file_path(f".{os.path.basename(self.path)}.tmp")
+            os.rename(path, self.path)
+        return True
+
+    def pickle(self, obj):
+        if self.file:
+            pickle.dump(obj, self.file, pickle.HIGHEST_PROTOCOL)
+
+    def unpickle(self):
+        if self.file:
+            return pickle.load(self.file)

curds/test_data.py

@@ -6,8 +6,14 @@ import xdg.BaseDirectory
 
 xdg_data_home = xdg.BaseDirectory.xdg_data_home
 testing_data  = os.path.join(xdg_data_home, "emmental-testing")
+testing_file  = os.path.join(testing_data, "test.file")
+testing_temp  = os.path.join(testing_data, ".test.file.tmp")
 
 class TestDataModule(unittest.TestCase):
+    def setUp(self):
+        if os.path.exists(testing_file):
+            os.remove(testing_file)
+
     def test_dir(self):
         self.assertEqual(data.emmental_data, testing_data)
         self.assertTrue(os.path.exists(testing_data))
@@ -17,3 +23,41 @@ class TestDataModule(unittest.TestCase):
         path = os.path.join(testing_data, "test")
         self.assertEqual(data.data_file_path("test"), path)
         self.assertFalse(os.path.exists(path))
+        self.assertEqual(data.READ, 'rb')
+        self.assertEqual(data.WRITE, 'wb')
+
+    def test_data_file_init(self):
+        f = data.DataFile("test.file", data.READ)
+        self.assertEqual(f.path, testing_file)
+        self.assertFalse(os.path.exists(testing_file))
+        self.assertEqual(f.mode, data.READ)
+        self.assertIsNone(f.file)
+
+        f = data.DataFile("test.file", data.WRITE)
+        self.assertFalse(os.path.exists(testing_file))
+        self.assertEqual(f.mode, data.WRITE)
+        self.assertIsNone(f.file)
+
+    def test_data_file_read_write(self):
+        test = [ "test", "saving", "a", "list" ]
+        with data.DataFile("test.file", data.READ) as f:
+            self.assertIsNone(f.file)
+            f.pickle(test)
+            self.assertIsNone(f.unpickle())
+
+        with data.DataFile("test.file", data.WRITE) as f:
+            self.assertIsNotNone(f.file)
+            self.assertEqual(f.file.name, testing_temp)
+            self.assertFalse(os.path.exists(testing_file))
+            self.assertTrue(os.path.exists(testing_temp))
+            f.pickle(test)
+
+        self.assertIsNone(f.file)
+        self.assertFalse(os.path.exists(testing_temp))
+        self.assertTrue(os.path.exists(testing_file))
+
+        with data.DataFile("test.file", data.READ) as f:
+            self.assertIsNotNone(f.file)
+            self.assertEqual(f.file.name, testing_file)
+            lst = f.unpickle()
+            self.assertEqual(test, lst)