按大小与计数进行汇总

size 和 count 的区别在于：

size 计算 NaN 值， count 没有。

df = pd.DataFrame(
        {"Name":["Alice", "Bob", "Mallory", "Mallory", "Bob" , "Mallory"],
         "City":["Seattle", "Seattle", "Portland", "Seattle", "Seattle", "Portland"],
         "Val": [4, 3, 3, np.nan, np.nan, 4]})

df
# Output: 
#        City     Name  Val
# 0   Seattle    Alice  4.0
# 1   Seattle      Bob  3.0
# 2  Portland  Mallory  3.0
# 3   Seattle  Mallory  NaN
# 4   Seattle      Bob  NaN
# 5  Portland  Mallory  4.0

df.groupby(["Name", "City"])['Val'].size().reset_index(name='Size')
# Output: 
#       Name      City  Size
# 0    Alice   Seattle     1
# 1      Bob   Seattle     2
# 2  Mallory  Portland     2
# 3  Mallory   Seattle     1

df.groupby(["Name", "City"])['Val'].count().reset_index(name='Count')
# Output: 
#       Name      City  Count
# 0    Alice   Seattle      1
# 1      Bob   Seattle      1
# 2  Mallory  Portland      2
# 3  Mallory   Seattle      0