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我正在编写一个函数，将字符串解析为另一个函数使用的列表。它执行的操作之一是将字符附加到（有时是深度递归）列表中特定递归深度（由名为 的变量定义的深度）的字符串lvl。这个操作是一个名为的函数listSurgery，它应该被一个索引列表调用，该列表指示下一个列表在前一个列表中的位置，最终索引告诉深度列表中的哪个索引执行操作，被调用索引的空白列表，我不知道为什么。它应该被调用的列表是[-1]，但调试显示它被调用了[]。这是代码，缩写：

def listAssign(lst,index,item):
    """Assigns item item to list lst at index index, returns modified list."""
    lst[index] = item
    return lst

def listInsert(lst,index,item):
    """Inserts item item to list lst at index index, returns modified list."""
    print "listInsert just got called with these arguments:",lst,index,item
    if index == 'end':
        index = len(lst)
    lst.insert(index,item)
    return lst

def listSurgery(lst,indices,f,*extraArgs):
    """Performs operation f on list lst at depth at indices indices, returns modified list."""
    print "listSurgery just got called with these arguments:",lst,indices,f,extraArgs
    parent = lst
    for index in indices[:-1]:
        parent = parent[index]
    parent = f(parent,indices[-1],*extraArgs)
    return listSurgery(lst,indices[:-1],listAssign,parent)

def parseStringToList(s):
    """Takes in a user-input string, and converts it into a list to be passed into parseListToExpr."""
    # ...
    l = [] # List to build from string; built by serially appending stuff as it comes up
    b = True # Bool for whether the parser is experiencing spaces (supposed to be True if last character processed was a space)
    t = False # Bool for whether the parser is experiencing a string of non-alphanumeric characters (supposed to be True if last character was a non-alphanumeric character)
    lvl = 0 # Keeps track of depth at which operations are supposed to be occurring
    for c in s:
        if c == ' ': # If c is a space, ignore it but send signal to break off any strings currently being written to
            b = True
        # Some elifs for c being non alphanumeric
        else: # If c is alphanumeric, append it to the string it's working on
            print c,"got passed as an alphanumeric; lvl is",lvl
            assert c.isalnum()
            if b or t: # If the string it's working on isn't alphanumeric or doesn't exist, append a new string
                l = listSurgery(l,[-1]*lvl + ['end'],listInsert,'')
                b, t = False, False
            l = listSurgery(l,[-1]*(lvl+1),lambda x,y,z:listAssign(x,y,x[y]+z),c)
        print l
    return l

while op != 'exit' and op != 'quit': # Keep a REPL unless the user types "exit" or "quit", in which case exit
    op = raw_input("> ")
    if op == 'help':
        pass # Print help stuff
    elif op in {'quit','exit'}:
        pass
    else:
        print str(parseStringToList(op))

python -tt code.py我用和 typed调用了代码1+1=2，这就是我得到的：

> 1+1=2
1 got passed as an alphanumeric; lvl is 0
listSurgery just got called with these arguments: [] ['end'] <function listInsert at 0x10e9d16e0> ('',)
listInsert just got called with these arguments: [] end 
listSurgery just got called with these arguments: [''] [] <function listAssign at 0x10e9d10c8> ([''],)
Traceback (most recent call last):
  File "analysis.py", line 276, in <module>
    print str(parseStringToList(op))
  File "analysis.py", line 218, in parseStringToList
    l = listSurgery(l,[-1]*lvl + ['end'],listInsert,'')
  File "analysis.py", line 63, in listSurgery
    return listSurgery(lst,indices[:-1],listAssign,parent)
  File "analysis.py", line 62, in listSurgery
    parent = f(parent,indices[-1],*extraArgs)
IndexError: list index out of range

谁能解释一下？为什么listSurgery得到[]而不是[-1]？lvl是0，此时应该传递的参数是[-1]*(lvl+1)。甚至不要介意为什么它被调用['']而不是'1'.

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1 回答 1

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你lvl是 0，所以[-1]*lvl + ['end']是['end']，并且indices[:-1]是['end'][:-1]。现在 ['end'] 是长度为 1 的列表，因此['end'][:-1]与 相同['end'][:1-1]，与 相同['end'][:0]。这评估为空列表。

于 2013-01-22T05:30:23.357 回答