*tuple 和 **dict 在 Python 中是什么意思? [英] What does *tuple and **dict mean in Python?
问题描述
正如 PythonCookbook 中提到的,*
可以添加在元组之前.*
在这里是什么意思?
第 1.18 章.将名称映射到序列元素:
from collections import namedtupleStock = namedtuple('Stock', ['name', 'shares', 'price'])s = 股票(*rec)# 这里rec是一个普通的元组,例如:rec = ('ACME', 100, 123.45)
在同一部分,**dict
表示:
from collections import namedtupleStock = namedtuple('Stock', ['name', 'shares', 'price', 'date', 'time'])# 创建原型实例stock_prototype = Stock('', 0, 0.0, None, None)# 将字典转换为股票的函数def dict_to_stock(s):返回 stock_prototype._replace(**s)
**
在这里的作用是什么?
在函数调用中
*t
表示将此可迭代对象的元素视为此函数调用的位置参数."
def foo(x, y):打印(x,y)>>>t = (1, 2)>>>脚)1 2
从 v3.5 开始,您也可以在列表/元组/集合文字中执行此操作:
<预><代码>>>>[1, *(2, 3), 4][1, 2, 3, 4]**d
表示将字典中的键值对视为此函数调用的附加命名参数."
def foo(x, y):打印(x,y)>>>d = {'x':1,'y':2}>>>食物)1 2
从 v3.5 开始,您也可以在字典文字中执行此操作:
<预><代码>>>>d = {'a': 1}>>>{'b': 2, **d}{'b':2,'a':1}在函数签名中
*t
表示将所有额外的位置参数带到这个函数,并将它们作为一个元组打包到这个参数中."
def foo(*t):打印(吨)>>>富(1, 2)(1, 2)
**d
的意思是将所有附加的命名参数带到这个函数中,并将它们作为字典条目插入到这个参数中."
def foo(**d):打印(四)>>>富(x=1,y=2){'y':2,'x':1}
在赋值和 for
循环中
*x
表示在右侧消耗额外的元素",但它不必是最后一项.请注意,x
将始终是一个列表:
请注意,出现在 *
之后的参数是仅限关键字的:
def f(a, *, b): ...f(1, b=2) # 没问题f(1, 2) # 错误:b 是关键字
Python3.8 添加了仅定位参数,表示不能用作关键字参数的参数.它们出现在 /
之前(*
上的双关语,位于仅关键字参数之前).
def f(a,/, p, *, k): ...f( 1, 2, k=3) # 没问题f( 1, p=2, k=3) # 很好f(a=1, p=2, k=3) # 错误:a 是位置唯一的
As mentioned in PythonCookbook, *
can be added before a tuple. What does *
mean here?
Chapter 1.18. Mapping Names to Sequence Elements:
from collections import namedtuple Stock = namedtuple('Stock', ['name', 'shares', 'price']) s = Stock(*rec) # here rec is an ordinary tuple, for example: rec = ('ACME', 100, 123.45)
In the same section, **dict
presents:
from collections import namedtuple Stock = namedtuple('Stock', ['name', 'shares', 'price', 'date', 'time']) # Create a prototype instance stock_prototype = Stock('', 0, 0.0, None, None) # Function to convert a dictionary to a Stock def dict_to_stock(s): return stock_prototype._replace(**s)
What is **
's function here?
In a function call
*t
means "treat the elements of this iterable as positional arguments to this function call."
def foo(x, y):
print(x, y)
>>> t = (1, 2)
>>> foo(*t)
1 2
Since v3.5, you can also do this in a list/tuple/set literals:
>>> [1, *(2, 3), 4]
[1, 2, 3, 4]
**d
means "treat the key-value pairs in the dictionary as additional named arguments to this function call."
def foo(x, y):
print(x, y)
>>> d = {'x':1, 'y':2}
>>> foo(**d)
1 2
Since v3.5, you can also do this in a dictionary literals:
>>> d = {'a': 1}
>>> {'b': 2, **d}
{'b': 2, 'a': 1}
In a function signature
*t
means "take all additional positional arguments to this function and pack them into this parameter as a tuple."
def foo(*t):
print(t)
>>> foo(1, 2)
(1, 2)
**d
means "take all additional named arguments to this function and insert them into this parameter as dictionary entries."
def foo(**d):
print(d)
>>> foo(x=1, y=2)
{'y': 2, 'x': 1}
In assignments and for
loops
*x
means "consume additional elements in the right hand side", but it doesn't have to be the last item. Note that x
will always be a list:
>>> x, *xs = (1, 2, 3, 4)
>>> x
1
>>> xs
[2, 3, 4]
>>> *xs, x = (1, 2, 3, 4)
>>> xs
[1, 2, 3]
>>> x
4
>>> x, *xs, y = (1, 2, 3, 4)
>>> x
1
>>> xs
[2, 3]
>>> y
4
>>> for (x, *y, z) in [ (1, 2, 3, 4) ]: print(x, y, z)
...
1 [2, 3] 4
Note that parameters that appear after a *
are keyword-only:
def f(a, *, b): ...
f(1, b=2) # fine
f(1, 2) # error: b is keyword-only
Python3.8 added positional-only parameters, meaning parameters that cannot be used as keyword arguments. They are appear before a /
(a pun on *
preceding keyword-only args).
def f(a, /, p, *, k): ...
f( 1, 2, k=3) # fine
f( 1, p=2, k=3) # fine
f(a=1, p=2, k=3) # error: a is positional-only
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