context

Author: qiwsir

235.md

@@ -253,4 +253,8 @@ contextlib.contextmanager是一个装饰器，它作用于生成器函数（也
 
 特别要提醒，被装饰的生成器函数只能产生一个值，否则就会抛出RuntimeError异常；如果有as子句，则所产生的值，会通过as子句赋给某个变量，就如同前面那样。
 
+$ python 23504.py
+before yield.
+the word is: contextmanager demo.
+after yield.
 

2code/23503.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+# coding=utf-8
+
+import cairo
+from contextlib import contextmanager
+
+@contextmanager
+def saved(cr):
+    cr.save()
+    try:
+        yield cr
+    finally:
+        cr.restore()
+
+def tree(angle):
+    cr.move_to(0, 0)
+    cr.translate(0, -65)
+    cr.line_to(0, 0)
+    cr.stroke()
+    cr.scale(0.72, 0.72)
+    if angle > 0.72:
+        for a in [-angle, angle]:
+            with saved(cr):
+                cr.rotate(a)
+                tree(angle * 0.75)
+
+surf = cairo.ImageSurface(cairo.FORMAT_ARGB32, 280, 204)
+cr = cairo.Context(surf)
+cr.translate(140, 203)
+cr.set_line_width(5)
+tree(0.75)
+surf.write_to_png('fractal-tree.png')

2code/23504.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python
+# coding=utf-8
+
+from contextlib import contextmanager
+
+@contextmanager
+def demo():
+    print "before yield."
+    yield "contextmanager demo."
+    print "after yield."
+
+with demo() as dd:
+    print "the word is: %s" % dd
+