python - 元组和列表的不同访问元素

Question

为什么在将 t 设为 tuple 时，在访问 t 的元素时存在这种差异？

>>> t = [('ID','int')]
>>> for r in t:
print r


('ID', 'int')


t = (('ID','int'))
>>> for r in t:
print r


ID
int

我希望这与第一个示例完全相同!而用多个元素填充元组时，行为会发生变化。

>>> t = (('ID','int'),('DEF','str'))
>>> for r in t:
print r


('ID', 'int')
('DEF', 'str')
>>> t = [('ID','int'),('DEF','str')]
>>> for r in t:
print r


('ID', 'int')
('DEF', 'str')

谁能给个简短的解释？我正在运行 python 2.7

Answer 1

(('a', 'b')) 与 ('a', 'b') 相同。

你实际上想要 (('a', 'b'),)

这在此处记录:

5.13. Expression lists

expression_list ::= expression ( "," expression )* [","]

An expression list containing at least one comma yields a tuple. The length of the tuple is the number of expressions in the list. The expressions are evaluated from left to right.

The trailing comma is required only to create a single tuple (a.k.a. a singleton); it is optional in all other cases. A single expression without a trailing comma doesn’t create a tuple, but rather yields the value of that expression. (To create an empty tuple, use an empty pair of parentheses: ().)

请记住，如果没有此限制，表达式 (3) * (4) 应该是两个数字或两个元组的乘积吗？大多数用户会期望它是数字的乘法。