Python 在解析表时处理 NoneType

Question

我正在尝试比较两个表( table_a 和 table_b )并减去 table_a 的最后一列从table_b的最后一列开始。但是，table_a 包含一个额外的行，导致我得到 NoneType错误。我是否仍然可以包含 table_a 中的“Plums”行？并输出 NULL对于德尔塔电池？下面是我的可测试代码。

当前代码:

from datetime import datetime
import itertools

table_a = (
      (datetime(2016, 9, 28, 0, 0), 'Apples', 650, 700, 850),
      (datetime(2016, 9, 28, 0, 0), 'Oranges', 900, 950, 1000),
      (datetime(2016, 9, 28, 0, 0), 'Grapes', 1050, 1100, 1150),
      (datetime(2016, 9, 28, 0, 0), 'Plums', 2000, 3000, 4000)
      )

table_b = (
      (datetime(2016, 9, 27, 0, 0), 'Apples', 50, 150, 200),
      (datetime(2016, 9, 27, 0, 0), 'Oranges', 250, 350, 400),
      (datetime(2016, 9, 27, 0, 0), 'Grapes', 450, 550, 600),
      )

table_format = '{:<10}|{:<8}|{:<8}|{:<8}|{:<8}|{:<12}'
line_sep = ('-' * 60)

print(line_sep)
print(table_format.format('Date', 'Count_1', 'Count_2', 'Count_3' , 'Count_4', 'Count_4_Delta'))


for a, b in itertools.zip_longest(table_a, table_b):
      l = str(a[0])[0:10]
      m = a[1]
      n = a[2]
      o = a[3]
      p = a[4]
      q = b[4]
      print(line_sep)
      print(table_format.format(l, m, n, o, p, (p-q)))

输出有错误:

------------------------------------------------------------
Date      |Count_1 |Count_2 |Count_3 |Count_4 |Count_4_Delta
------------------------------------------------------------
2016-09-28|Apples  |650     |700     |850     |650         
------------------------------------------------------------
2016-09-28|Oranges |900     |950     |1000    |600         
------------------------------------------------------------
2016-09-28|Grapes  |1050    |1100    |1150    |550         
Traceback (most recent call last):
  File "/media/test.py", line 30, in <module>
    q = b[4]
TypeError: 'NoneType' object is not subscriptable

如果我添加 if 语句来删除 NoneType，它会打印表格而不会出现错误，但会排除“Plums”行。

for a, b in itertools.zip_longest(table_a, table_b):
      if a and b is not None:
            l = str(a[0])[0:10]
            m = a[1]
            n = a[2]
            o = a[3]
            p = a[4]
            q = b[4]
            print(line_sep)
            print(table_format.format(l, m, n, o, p, (p-q)))

带有 If 语句的输出:

------------------------------------------------------------
Date      |Count_1 |Count_2 |Count_3 |Count_4 |Count_4_Delta
------------------------------------------------------------
2016-09-28|Apples  |650     |700     |850     |650         
------------------------------------------------------------
2016-09-28|Oranges |900     |950     |1000    |600         
------------------------------------------------------------
2016-09-28|Grapes  |1050    |1100    |1150    |550              

我想要以下输出。其中“Plums”行仍会打印，但增量单元格具有字符串“NULL”。

期望的输出:

------------------------------------------------------------
Date      |Count_1 |Count_2 |Count_3 |Count_4 |Count_4_Delta
------------------------------------------------------------
2016-09-28|Apples  |650     |700     |850     |650         
------------------------------------------------------------
2016-09-28|Oranges |900     |950     |1000    |600         
------------------------------------------------------------
2016-09-28|Grapes  |1050    |1100    |1150    |550          
------------------------------------------------------------
2016-09-27|Plums   |2000    |3000    |4000    |NULL        

Answer 1

itertools.zip_longest接受可选的 fillvalue 参数。如果提供了，则使用它来代替 None:

>>> list(itertools.zip_longest([1, 2, 3], [4, 5]))
[(1, 4), (2, 5), (3, None)]
>>> list(itertools.zip_longest([1, 2, 3], [4, 5], fillvalue='NULL'))
[(1, 4), (2, 5), (3, 'NULL')]

您可以提供空行(NULL 值列表)作为 fillvalue:

class EmptyValue:
    def __sub__(self, other):
        return 'NULL'
    def __rsub__(self, other):
        return 'NULL'

empty_row = [None, 'NULL', EmptyValue(), EmptyValue(), EmptyValue()]
for a, b in itertools.zip_longest(table_a, table_b, fillvalue=empty_row):
    ...