python - 我该如何修复 : "TypeError: cannot unpack non-iterable NoneType object"

Question

我一直在通过 stackoverflow 和其他各种网站进行搜索，但我已经有大约一个星期无法解决这个错误了。

我正在尝试获取字典中每个国家/地区的最小值和最大值。字典的关键字是区域。我不确定类型错误在哪里，但如果有人能提供帮助，我将不胜感激。

这是错误:

        min_tup, max_tup = get_min_max(D,region,option)
  File "proj08.py", line 107, in get_min_max
    return min[0], max[0]
UnboundLocalError: local variable 'min' referenced before assignment

这是示例输入:地区，选项:北美，2

这是解释函数和 .csv 的文档 https://www.cse.msu.edu/~cse231/Online/Projects/Project08/Project08.pdf https://www.cse.msu.edu/~cse231/Online/Projects/Project08/data_short.csv

代码如下:

import csv
from operator import itemgetter
# do NOT import sys

REGION_LIST = ['East Asia & Pacific',
           'Europe & Central Asia',
           'Latin America & Caribbean',
           'Middle East & North Africa',
           'North America',
           'South Asia',
           'Sub-Saharan Africa']

PROMPT = "\nSpecify a region from this list or 'q' to quit -- \nEast 
Asia & Pacific,Europe & Central Asia,Latin America & Caribbean,Middle 
East & North 
Africa,North America,South Asia,Sub-Saharan Africa: "


def open_file():
# Opens a file
while True:
    try:
        file = input("Input a file: ")
        fp = open(file, "r")
        return fp
    except FileNotFoundError:
        print("Invalid filename, please try again.")


def read_file(fp):

# Sets read Csv file to a variable
reader = csv.reader(fp)
# Skips the header
next(reader, None)

# Country List
country_list = []
# sets a dictionary
Dict = dict()
for line in reader:
    try:
        skipper = ""
        if skipper in line:
            continue
        else:
            region = line[6]
            country = line[0].strip()
            electricty = float(line[2])
            fertility = float(line[3])
            gdp = float(line[4])
            life_expectancy = float(line[5])

            country_list = [country, electricty, fertility, GDP, 
life_expectancy]

            if region in Dict.keys():
                Dict[region].append(country_list)
            elif region not in Dict.keys():
                Dict[region] = [country_list]
            else:
                continue
                
    except KeyError:
        continue
    except ValueError:
        continue

return Dict


def get_min_max(Dict, region, option):
lis = []
for k, v in Dict.items():
    if region in k[0]:
        if option == 1:
            electricity = v[1]

            tup = tuple(k, electricity)
            lis.append(tup)

            min = sorted(lis, key=itemgetter(1))
            max = sorted(lis, key=itemgetter(1), reverse=True)

        if option == 2:
            fertility = v[2]

            tup = tuple(k, fertility)
            lis.append(tup)

            min = sorted(lis, key=itemgetter(1))
            max = sorted(lis, key=itemgetter(1), reverse=True)

        if option == 3:
            gdp = v[3]

            tup = tuple(k, gdp)
            lis.append(tup)

            min = sorted(lis, key=itemgetter(1))
            max = sorted(lis, key=itemgetter(1), reverse=True)

        if option == 4:
            life_expectancy = v[4]

            tup = tuple(k, life_expectancy)
            lis.append(tup)
            min = sorted(lis, key=itemgetter(1))
            max = sorted(lis, key=itemgetter(1), reverse=True)

return min[0], max[0]


def display_all_countries(D, region):

if region in REGION_LIST:

    if region == 'all':
        print("\nDisplaying {} Region:\n".format(region))
        print("{:32s}{:>20s}{:>20s}{:>17s}{:>18s}".format(
            "Country", "Electricity Access", "Fertility rate", "GDP 
per capita", "Life expectancy"))
        for k, v in D.items():
            if region in v[0]:
                country = v[0]
                electricity = v[1]
                fertility = v[2]
                gdp = v[3]
                life = v[4]
                tup = (country, electricity, fertility, gdp, life)

                sorted(tup, key=itemgetter(0), reverse=True)
                print("{:32s}{:>20.2f}{:>20.2f}{:>17.2f} 
{:>18.2f}".format(
                    tup[0], tup[1], tup[2], tup[3], tup[4]))

if region not in REGION_LIST:
    return None


def get_top10(D):
    pass


def display_options():
    """
    DO NOT CHANGE
    Display menu of options for program
    """
    OPTIONS = """\nMenu
    1: Minimum and Maximum Countries Access to Electricity
    2: Minimum and Maximum Countries Fertility Rate
    3: Minimum and Maximum Countries GDP per Capita
    4: Minimum and Maximum Countries Life Expectancy
    5: List of countries in a region
    6: Top 10 Countries in the world by GDP per Capita\n"""
    print(OPTIONS)


def main():
    file = open_file()
    # while True:
    #     if user == 'East Asia & Pacific' or user == 'Europe & 
    Central Asia' or user == 'Middle East & North Africa' or user == 
   'Latin America & Caribbean' or user == 'North America' or user == 
   'South Asia' or user == 'Sub-Saharan Africa':
#         print("\nRegion: ".format(user))
#         display_options()

#     if user == "Q" or user == "q":
#         break
#     else:
#         user = input(PROMPT)
region = 'North America'
option = '2'
superD = read_file(file)
mina = get_min_max(superD, region, option)

#print(mina)

if __name__ == '__main__':
    main()

Answer 1

错误是告诉你不能使用解包赋值，例如

x, y = function()

因为该函数返回了一些无法解压的东西(在本例中为无)这意味着您的函数以某种方式返回 None 。如果没有可重用的示例，我们不能肯定地说，但我猜这是因为函数中的第一个 if 条件可以返回 None。

虽然允许，但在 python 函数中有多个不同的返回类型通常不是一个好主意。这是因为调用者必须知道如何处理函数可能做的不同事情，而不是能够相信函数会工作并给他们一个好的答案(当然，假设他们使用的是正确的输入。)