Python编程中一定要注意的那些“坑”（一）

1 逗号不是运算符，只是个普通的分隔符 >>> x = 3, 5 >>> x (3, 5) >>> x == 3, 5 (False, 5) >>> 1, 2, 3 (1, 2, 3) >>> 3 in [1, 2, 3], 5 (True, 5)

2 ++和--也不是运算符，虽然有时候这样用也行 >>> x = 3 >>> x+++5 8 >>> x++ SyntaxError: invalid syntax >>> ++5 5 >>> ++++++++5 5 >>> --5 5

# 下面这个代码是上面那个代码的等价形式

>>> -(-5) 5 >>> ---------5 -5

3 lambda表达式中变量的作用域 >>> d = dict() # 这里有个坑 >>> for i in range(5): d[i] = lambda :i**2 >>> d[2]() 16 >>> d[3]() 16 # 这样看的更清楚一些 # lambda表达式中i的值是调用时决定的 >>> i = 10 >>> d[0]() 100 # 写成下面这样子就没问题了 >>> d = dict() >>> for i in range(5): d[i] = lambda x=i:x**2 >>> d[2]() 4 >>> d[3]() 9

4 某个作用域中只要有某变量的赋值语句，它就是个局部变量 >>> x = 10 >>> def demo(): print(x) # 这样是可以的，访问全局变量 >>> demo() 10 >>> def demo(): print(x) x = 3 print(x) # 这样是错的，x是局部变量，在x=3之前不存在x，print()失败 >>> demo() Traceback (most recent call last): File "<pyshell#156>", line 1, in <module> demo() File "<pyshell#155>", line 2, in demo print(x) UnboundLocalError: local variable 'x' referenced before assignment

5 纠结的元组到底可变不可变 >>> x = (1, 2, 3) # 元组中的元素不可修改 >>> x[0] = 4 Traceback (most recent call last): File "<pyshell#161>", line 1, in <module> x[0] = 4 TypeError: 'tuple' object does not support item assignment >>> x = ([1, 2], 3) # 不能修改元组中的元素值 >>> x[0] = [3] Traceback (most recent call last): File "<pyshell#163>", line 1, in <module> x[0] = [3] TypeError: 'tuple' object does not support item assignment >>> x ([1, 2], 3) >>> x[0] = x[0] + [3] Traceback (most recent call last): File "<pyshell#165>", line 1, in <module> x[0] = x[0] + [3] TypeError: 'tuple' object does not support item assignment >>> x ([1, 2], 3) # 这里有个坑，虽然显示操作失败了，但实际上成功了 >>> x[0] += [3] Traceback (most recent call last): File "<pyshell#167>", line 1, in <module> x[0] += [3] TypeError: 'tuple' object does not support item assignment >>> x ([1, 2, 3], 3) >>> x[0].append(4) >>> x ([1, 2, 3, 4], 3) # y和x[0]指向同一个列表，通过其中一个可以影响另一个 >>> y = x[0] >>> y += [5] >>> x ([1, 2, 3, 4, 5], 3) # 执行完下面的语句，y和x[0]不再是同一个对象 >>> y = y + [6] >>> x ([1, 2, 3, 4, 5], 3) >>> y [1, 2, 3, 4, 5, 6]

6 字符串转换成数字的几种方式

>>> eval('9.9') 9.9 >>> eval('09.9') 9.9 >>> float('9.9') 9.9 >>> float('09.9') 9.9 >>> int('9') 9 >>> int('09') 9

# 坑来了，使用eval()转换整数时前面不能有0

>>> eval('09') Traceback (most recent call last): File "<pyshell#187>", line 1, in <module> eval('09') File "<string>", line 1 09 ^ SyntaxError: invalid token