一文讲清Python参数解包之*按位与** 按名

def print_vector(x, y, z):
    print('<%s, %s %s>' % (x, y, z))

print_vector(1, 0, 1)
# <1, 0 1>

tuple_vec = (2, 0, 2)
list_vec = [2, 0, 2]

print_vector(tuple_vec[0], tuple_vec[1], tuple_vec[2])
# <2, 0 2>

print_vector(*tuple_vec)
# <2, 0 2>

print_vector(*list_vec)
# <2, 0 2>

genexpr = (x * x for x in range(3))
print_vector(*genexpr)
# <0, 1 4>

dict_vec = {'y': 0, 'z': 3, 'x': 1}
print_vector(**dict_vec)
# <1, 0 3>

print_vector(*dict_vec)
# <y, z x>

'<%s, %s %s>' % (1, 0, 1)      # <1, 0 1>
'<%s, %s, %s>' % (1, 0, 1)     # <1, 0, 1>

print_vector(*(2, 0, 2))              # <2, 0, 2>   tuple
print_vector(*[2, 0, 2])              # <2, 0, 2>   list
print_vector(*'abc')                  # <a, b, c>   str 逐字符
print_vector(*(i for i in (2, 0, 2)))# <2, 0, 2>   生成器
print_vector(*{'a': 1, 'b': 2, 'c': 3})  # <a, b, c>  dict 取键
print_vector(*range(1, 4))            # <1, 2, 3>   range
print_vector(*{7, 8, 9})              # <8, 9, 7>   set（顺序不确定）
print_vector(*counter("aabbc"))       # <a, b, c>   counter

syntaxerror: invalid syntax. perhaps you forgot a comma?
  出错行: 'genexpr = (x * xfor x in range(3))\n'
  列偏移: 12

import tokenize, io
for tok in tokenize.generate_tokens(io.stringio("x * xfor for").readline):
    ...
# name  'x'
# op    '*'
# name  'xfor'      <-- 注意这里
# name  'for'

(x * xfor x in range(3))   # syntaxerror: invalid syntax. perhaps you forgot a comma?
(x * x for x in range(3))  # ok
(x *x for x in range(3))   # ok   —— * 与名字紧贴不影响
(x* xfor x in range(3))    # syntaxerror —— 只把空格加在 * 左边没用

print_vector(**{'y': 0, 'z': 3, 'x': 1})   # <1, 0 3>
print_vector(**{'x': 1, 'y': 0, 'z': 3})   # <1, 0 3>
print_vector(**{'z': 3, 'x': 1, 'y': 0})   # <1, 0 3>

print_vector(*{'y': 0, 'z': 3, 'x': 1})    # <y, z x>
print_vector(*{'x': 1, 'y': 0, 'z': 3})    # <x, y z>

print_vector(*(1, 2, 3, 4))
# typeerror: print_vector() takes 3 positional arguments but 4 were given

print_vector(*(1, 2))
# typeerror: print_vector() missing 1 required positional argument: 'z'

print_vector(**{'a': 1, 'b': 2, 'c': 3})
# typeerror: print_vector() got an unexpected keyword argument 'a'

print_vector(**{'x': 1, 'y': 2})
# typeerror: print_vector() missing 1 required positional argument: 'z'

print_vector(**{'x': 1, 'y': 2}, **{'z': 3, 'x': 9})
# typeerror: print_vector() got multiple values for keyword argument 'x'

print_vector(**{1: 'a', 2: 'b', 3: 'c'})
# typeerror: keywords must be strings

print_vector(1, **{'x': 2, 'y': 0, 'z': 3})
# typeerror: print_vector() got multiple values for argument 'x'

print_vector(*5)
# typeerror: print_vector() argument after * must be an iterable, not int

def show(a, b, c):
    ...

show(**{'x': box, 'y': box, 'z': box})
# typeerror: show() got an unexpected keyword argument 'x'

def show(x, y, z):
    ...

show(*[box, box, box])                  # 形参同一性: (true, true, true)
show(**{'x': box, 'y': box, 'z': box})  # 形参同一性: (true, true, true)

def takes(**kw):
    return kw

takes(**{"a-b": 1})   # {'a-b': 1}   —— 合法，因为函数用 **kw 兜底

gen = (i for i in range(3))
print_vector(*gen)   # <0, 1 2>
print_vector(*gen)   # typeerror: print_vector() missing 3 required positional arguments: 'x', 'y', and 'z'

gen2 = iter([1, 2, 3])
print_vector(*gen2)  # <1, 2 3>
print_vector(*gen2)  # typeerror: print_vector() missing 3 required positional arguments: 'x', 'y', and 'z'

print_vector(*list_vec)  # <2, 0 2>
print_vector(*list_vec)  # <2, 0 2>

def consume(*args):
    return len(args)

def gen_n(n):
    for i in range(n):
        yield i

tracemalloc.start()
consume(*gen_n(1_000_000))
cur, peak = tracemalloc.get_traced_memory()

解包 1,000,000 个元素的生成器 -> 收到 1000000 个位置参数
峰值内存: 45.8 mib
解包 range(1_000_000) -> 收到 1000000 个位置参数，峰值内存: 45.8 mib

total = 0
for i in gen_n(1_000_000):
    total += i

解包 1,000,000 元素 list -> 收到 1000000 个参数
解包 10,000,000 元素 list -> 收到 10000000 个参数

def target(a, b, c):
    return a + b + c

def via_star(args):
    return target(*args)

lambda 内直接调用 target(1,0,1):     38.7 ns/次
经中间函数 target(*args)        :     59.3 ns/次
lambda 内原地 target(*args)     :     43.8 ns/次

load_global      1 (null + f)
load_const       1 (1)
load_const       2 (0)
load_const       1 (1)
precall          3
call             3          <-- 定长调用
return_value

load_global      1 (null + f)
load_const       1 ((1, 0, 1))
call_function_ex 0          <-- 变长调用，0 表示无关键字参数
return_value

load_global      1 (null + f)
load_const       4 (())
build_map        0
load_const       1 (1)
load_const       2 (0)
load_const       1 (1)
load_const       3 (('a', 'b', 'c'))
build_const_key_map  3
dict_merge       1          <-- 合并关键字字典
call_function_ex 1
return_value

load_global      1 (null + f)
load_fast        0 (args)
build_map        0
load_fast        1 (kw)
dict_merge       1
call_function_ex 1
return_value

a, b = [1], (0,)
print_vector(*a, *b, *[1])      # <1, 0 1>
print_vector(*[1], 0, 1)        # <1, 0 1>   * 之后还能跟普通位置参数
print_vector(1, **{'y': 0, 'z': 3})   # <1, 0 3>

{**{'x': 1}, **{'y': 0, 'z': 3}}   # {'x': 1, 'y': 0, 'z': 3}
[*[1], *(0,), *[1]]                # [1, 0, 1]
{*[1, 2], *(2, 3)}                 # {1, 2, 3}  —— 去重

print_vector(1, **{'y': 0, 'z': 3})   # ok
print_vector(1, **{'x': 2, 'y': 0, 'z': 3})
# typeerror: print_vector() got multiple values for argument 'x'   （实测）

def kwonly(a, *, b):
    return a + b

kwonly(1, **{'b': 2})   # 3
kwonly(1, 2)            # typeerror: kwonly() takes 1 positional argument but 2 were given

def wrapper(*args, **kwargs):     # 收集
    ...
    return func(*args, **kwargs)  # 摊开

def connect(host='127.0.0.1', port=8080, timeout=30, retry=3):
    return "host=%s port=%s timeout=%s retry=%s" % (host, port, timeout, retry)

defaults = {'host': '127.0.0.1', 'port': 8080, 'timeout': 30, 'retry': 3}
override = {'port': 443, 'timeout': 5}

默认配置        : host=127.0.0.1 port=8080 timeout=30 retry=3
合并后配置      : host=127.0.0.1 port=443 timeout=5 retry=3
字面量合并结果  : {'host': '127.0.0.1', 'port': 443, 'timeout': 5, 'retry': 3}

connect(host='0.0.0.0', **defaults)
# typeerror: connect() got multiple values for keyword argument 'host'   （实测）

connect(**{**defaults, 'host': '0.0.0.0'})   # 正确姿势

'%s' % 1        # '1'
'%s' % (1,)     # '1'
'%s' % (1, 2)   # typeerror: not all arguments converted during string formatting
'%(x)s' % {'x': 1}   # '1'

# —— 调用侧 ——
f(*iterable)          # 按位展开：tuple/list/str/生成器/range/dict(取键)/set(顺序不定)
f(**mapping)          # 按名展开：键必须为 str，顺序无关
f(*a, *b, **c, **d)   # pep 448，3.5+ 可多处解包
f(x, *a)              # 解包之后仍可跟普通位置参数

# —— 定义侧 ——
def f(*args, **kwargs)          # 打包：收集为 tuple / dict
def f(a, *, b)                  # b 为 keyword-only，只能由 ** 或 b= 提供

# —— 字面量 ——
[*a, *b]     # 列表拼接      (*a, *b)  # 元组拼接
{*a, *b}     # 集合合并去重  {**a, **b} # 字典合并，右侧覆盖

# —— 安全边界 ——
list/tuple/str/range/dict : 可重复解包
生成器 / map / filter / zip / iter() : 解包一次即耗尽
解包 = 全量物化，大流先 list 再决定是否解包
解包只传引用，不拷贝元素