python合并字典的2种方式

亦凉 2022-10-21 01:29 180阅读 0赞

python合并字典2种方式分别为，使用内置方法 update，和关键字参数操作

# 一 内置方法 #

a = {1:2}
    b = {3:4}
    
    
    a.update(b)
    print(a)

# 二 \*\*a操作-关键字参数 #

a = {1:2}
    b = {3:4}
    
    
    c = {**a,**b}
    print(c)

第一种是改变a字典

第二种是生成新的字典c。

补充说明： \*params 指讲所有值放置在同一个元组中。

# 三 进一步思考 #

\*\*a是什么？

从下面资料不难看出，用于函数调用时的传参和构建字典。

# 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

[positional-only parameters]: https://docs.python.org/3/whatsnew/3.8.html#positional-only-parameters