动态响应解包赋值语句

Question

在解包赋值语句中，被赋值的对象可以检查它被赋值的变量的数量吗？

class MyObject:

    def __iter__(self):
        n = some_diabolical_hack()
        print(f"yielding {n} vals")
        return iter(["potato"]*n)

类似：

>>> x, y = MyObject()
yielding 2 vals
>>> a, b, c = MyObject()
yielding 3 vals

在更一般的情况下，它是否可以内省 target_list 在作业中使用的“形状”？

>>> first, *blob, d[k], (x, y), L[3:7], obj.attr, last = MyObject()
unpacking to <_ast.Tuple object at 0xcafef00d>

潜在用例示例：改进的 MagicMock() 在用于修补右侧的某些对象时不需要预先配置固定的迭代长度赋值语句。

Answer 1

你可以使用回溯模块：

import traceback

def diabolically_invoke_traceback():
    call = traceback.extract_stack()[-2]
    print call[3]
    unpackers = call[3].split('=')[0].split(',')
    print len (unpackers)
    return range(len(unpackers))

In [63]: a, b, c = diabolically_invoke_traceback()
a, b, c = diabolically_invoke_traceback()
3

In [64]: a
Out[64]: 0

In [65]: b
Out[65]: 1

In [66]: c
Out[66]: 2

Answer 2

（免责声明：我不建议在生产质量代码中使用恶魔般的技术。此答案中的所有内容可能无法在与我不同的计算机、与我不同的 Python 版本或非 CPython 发行版上运行，明天早上可能就不行了。）

也许您可以通过检查调用帧的字节码来做到这一点。如果我正确阅读了bytecode guide，则多个分配由指令UNPACK_SEQUENCE 或UNPACK_EX 处理，具体取决于目标列表是否具有星号名称。这两条指令都在其参数中提供了有关目标列表形状的信息。

您可以编写您的恶魔函数来爬升帧层次结构，直到找到调用帧，并检查出现在代表赋值右侧的FUNCTION_CALL 之后的字节码指令。 （这是假设您对MyObject() 的调用是语句右侧的唯一内容）。然后你可以从指令的参数中提取目标列表大小并返回它。

import inspect
import dis
import itertools

def diabolically_retrieve_target_list_size():
    #one f_back takes us to `get_diabolically_sized_list`'s frame. A second one takes us to the frame of the caller of `get_diabolically_sized_list`.
    frame = inspect.currentframe().f_back.f_back
    #explicitly delete frame when we're done with it to avoid reference cycles.
    try:
        #get the bytecode instruction that immediately follows the CALL_FUNCTION that is executing right now
        bytecode_idx = frame.f_lasti // 2
        unresolved_bytecodes = itertools.islice(dis.get_instructions(frame.f_code), bytecode_idx+1, bytecode_idx+3)
        next_bytecode = next(unresolved_bytecodes)
        if next_bytecode.opname == "UNPACK_SEQUENCE":   #simple multiple assignment, like `a,b,c = ...`
            return next_bytecode.arg
        elif next_bytecode.opname == "EXTENDED_ARG":    #multiple assignment with splat, like `a, *b, c = ...`
            next_bytecode = next(unresolved_bytecodes)
            if next_bytecode.opname != "UNPACK_EX":
                raise Exception(f"Expected UNPACK_EX after EXTENDED_ARG, got {next_bytecode.opname} instead")
            args_before_star = next_bytecode.arg % 256
            args_after_star = next_bytecode.arg >> 8
            return args_before_star + args_after_star
        elif next_bytecode.opname in ("STORE_FAST", "STORE_NAME"): #single assignment, like `a = ...`
            return 1
        else:
            raise Exception(f"Unrecognized bytecode: {frame.f_lasti} {next_bytecode.opname}")
    finally:
        del frame

def get_diabolically_sized_list():
    count = diabolically_retrieve_target_list_size()
    return list(range(count))

a,b,c = get_diabolically_sized_list()
print(a,b,c)
d,e,f,g,h,i = get_diabolically_sized_list()
print(d,e,f,g,h,i)
j, *k, l = get_diabolically_sized_list()
print(j,k,l)
x = get_diabolically_sized_list()
print(x)

结果：

0 1 2
0 1 2 3 4 5
0 [] 1
[0]