Add with statement feature (#58)

* Add with statement feature

* Update Python.swift

* Update PythonRuntimeTests.swift

* Update Python.swift

* Update Python.swift

* Update PythonRuntimeTests.swift

Author: philipturner

PythonKit/Python.swift

@@ -721,6 +721,15 @@ public struct PythonInterface {
     public var versionInfo: PythonObject {
         return self.import("sys").version_info
     }
+    
+    /// Emulates a Python `with` statement.
+    /// - Parameter object: A context manager object.
+    /// - Parameter body: A closure to call on the result of `object.__enter__()`.
+    public func with(_ object: PythonObject, _ body: (PythonObject) throws -> Void) rethrows {
+        let yieldValue = object.__enter__()
+        try body(yieldValue)
+        yieldValue.__exit__()
+    }
 }
 
 //===----------------------------------------------------------------------===//

Tests/PythonKitTests/PythonRuntimeTests.swift

@@ -334,4 +334,23 @@ class PythonRuntimeTests: XCTestCase {
         }
         XCTAssertEqual(bytes, otherBytes)
     }
+    
+    /// Tests an emulation of the Python `with` statement.
+    ///
+    /// Mirrors:
+    /// ```python
+    /// with open('temp', 'w') as opened_file:
+    ///      opened_file.write('Contents')
+    /// with open('temp', 'r') as opened_file:
+    ///      contents = opened_file.read()
+    /// ```
+    func testWith() {
+        Python.with(Python.open("temp", "w")) { opened_file in
+            opened_file.write("Contents")
+        }
+        Python.with(Python.open("temp", "r")) { opened_file in
+            let contents = opened_file.read()
+            XCTAssertEqual("Contents", String(contents)!)
+        }
+    }
 }