因为timedelta定义如下：

class datetime.timedelta([days,] [seconds,] [microseconds,] [milliseconds,] [minutes,] [hours,] [weeks])

All arguments are optional and default to 0.

你可以轻松地说“三天和四毫秒”与可选参数的方式。

>>> datetime.timedelta(days=3, milliseconds=4)

datetime.timedelta(3, 0, 4000)

>>> datetime.timedelta(3, 0, 0, 4) #no need for that.

datetime.timedelta(3, 0, 4000)

而对于str转换，它返回一个不错的格式化值，而不是__repr__。用于提高可读性。来自文档：

str(t) Returns a string in the form [D day[s], ][H]H:MM:SS[.UUUUUU],

where D is negative for negative t. (5)

>>> datetime.timedelta(seconds = 42).__repr__()

'datetime.timedelta(0, 42)'

>>> datetime.timedelta(seconds = 42).__str__()

'0:00:42'

Checkout文档：