TypeError：類型為'NoneType'的對象沒有len（）python

Question

我不斷收到這個錯誤

TypeError: object of type 'NoneType' has no len()

這是代碼：

def main():
    myList = [ ]
    myList = read_csv()
    ## myList = showList(myList)
    searchList = searchQueryForm(myList)
    if len(searchList) == 0:
        print("I have nothing to print")
    else:
        showList(searchList)

Answer 1

如果沒有找到， searchQueryForm顯然會返回None 。 由於您不能將len應用於None ，因此必須明確檢查：

if searchList is None or len(searchList) == 0:

Answer 2

您要從中獲取len()對象顯然是None對象。

這是searchList ，從返回searchQueryForm(myList) 。

因此，當它不應該是時為None 。

修復該函數或保留其可以返回None的事實：

if len(searchlist or ()) == 0:

要么

if not searchlist:

Answer 3

searchQueryForm()函數返回None值，並且len()內置函數不接受None類型參數。 因此TypeError異常。

演示 ：

>>> searchList = None
>>> print type(searchList)
<type 'NoneType'>
>>> len(searchList)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
TypeError: object of type 'NoneType' has no len()

在if循環中添加一個條件以檢查searchList是否為None

演示 ：

>>> if searchList==None or len(searchList) == 0:
...   print "nNothing"
... 
nNothing

如果代碼沒有進入最后的if loop ，則searchQueryForm()函數中缺少return語句。 默認情況下， None值返回時，我們沒有從函數返回任何特定的值。

def searchQueryForm(alist):
    noforms = int(input(" how many forms do you want to search for? "))
    for i in range(noforms):
        searchQuery = [ ]
        nofound = 0 ## no found set at 0
        formname = input("pls enter a formname >> ") ## asks user for formname
        formname = formname.lower() ## converts to lower case
        for row in alist:
            if row[1].lower() == formname: ## formname appears in row2
                searchQuery.append(row) ## appends results
                nofound = nofound + 1 ## increments variable
                if nofound == 0:
                    print("there were no matches")
                    return searchQuery
    return []
   # ^^^^^^^    This was missing