TypeError：类型为'NoneType'的对象没有len（）python

Question

我不断收到这个错误

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

这是代码：

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

如果没有找到， searchQueryForm显然会返回None 。 由于您不能将len应用于None ，因此必须明确检查：

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

Answer 2

您要从中获取len()对象显然是None对象。

这是searchList ，从返回searchQueryForm(myList) 。

因此，当它不应该是时为None 。

修复该函数或保留其可以返回None的事实：

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

要么

if not searchlist:

Answer 3

searchQueryForm()函数返回None值，并且len()内置函数不接受None类型参数。 因此TypeError异常。

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循环中添加一个条件以检查searchList是否为None

演示 ：

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

如果代码没有进入最后的if loop ，则searchQueryForm()函数中缺少return语句。 默认情况下， None值返回时，我们没有从函数返回任何特定的值。

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