在解包賦值語句中，被賦值的對象能否檢查它被賦值的變量數量？class MyObject: def __iter__(self): n = some_diabolical_hack() print(f"yielding {n} vals") return iter(["potato"]*n)就像是：>>> x, y = MyObject()yielding 2 vals>>> a, b, c = MyObject()yielding 3 vals在更一般的情況下，它可以內省target_list分配中使用的“形狀”嗎？>>> first, *blob, d[k], (x, y), L[3:7], obj.attr, last = MyObject()unpacking to <_ast.Tuple object at 0xcafef00d>示例潛在用例：MagicMock()當用于修補賦值語句右側的某個對象時，不需要預先配置固定迭代長度的改進。
2 回答
胡子哥哥
TA貢獻1825條經驗 獲得超6個贊
您可以使用回溯模塊：
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
慕勒3428872
TA貢獻1848條經驗 獲得超6個贊
也許您可以通過檢查調用幀的字節碼來做到這一點。如果我正確閱讀了字節碼指南，多重賦值由指令UNPACK_SEQUENCE或處理UNPACK_EX，具體取決于目標列表是否具有帶星號的名稱。這兩個指令都在其參數中提供了有關目標列表形狀的信息。
您可以編寫您的惡魔函數來爬升框架層次結構，直到找到調用框架，并檢查在FUNCTION_CALL代表賦值右側的之后發生的字節碼指令。（這是假設您調用的MyObject()是語句右側的唯一內容）。然后您可以從指令的參數中提取目標列表大小并返回它。
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)
結果：
0 1 2
0 1 2 3 4 5
0 [] 1
[0]
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