Update 31.md

jackzhenguo · web-flow · commit 851770759eaf · 2021-08-31T22:55:36.000+08:00
diff --git a/md/31.md b/md/31.md
@@ -9,10 +9,37 @@
 返回一个set对象，集合内不允许有重复元素：
 
 ```python
-In [159]: a = [1,4,2,3,1]
+In [1]: a = [1,4,2,3,1]
 
-In [160]: set(a)
-Out[160]: {1, 2, 3, 4}
+In [2]: set(a)
+Out[2]: {1, 2, 3, 4}
+
+In [3]: b = set(a)
+
+In [4]: b.add(5)
+
+In [5]: b
+Out[5]: {1, 2, 3, 4, 5}
+
+In [6]: b.pop()
+Out[6]: 1
+
+In [7]: b
+Out[7]: {2, 3, 4, 5}
+
+In [8]: b.pop()
+Out[8]: 2
+
+In [9]: b
+Out[9]: {3, 4, 5}
+
+# 注意pop删除集合内任意一个元素
+In [10]: help(b.pop)
+Help on built-in function pop:
+
+pop(...) method of builtins.set instance
+    Remove and return an arbitrary set element.
+    Raises KeyError if the set is empty.
 ```
 
-<center>[上一个例子](30.md)    [下一个例子](32.md)</center>
+<center>[上一个例子](30.md)    [下一个例子](32.md)</center>
