Pandas - How to make a groupment in which a new column is the result of (sum of a column)/(number of itens grouped)?

Question

I need to make some kind of groupment in which a new column (result) is the sum of the values column divided by number of items found? Could anyone help me, please?

For example:

Table A

+-------+------+
| item  | value|
+-------+------+
| x     |  100 |
| y     |  200 |
| y     |  400 | 
+-------+------+

Correct Result:

Table B

+-------+-----------+
| item  | result    |
+-------+-----------+
| x     | 100/1     |
| y     |(200+400)/2|
+-------+-----------+

Code:

d = {'item': ['x', 'y', 'y'], 'value': [100,200,400]}
df = pd.DataFrame(data=d)
df

Answer 1

You can use DataFrameGroupBy.agg:

s = df.groupby('item')['value'].agg(lambda x: x.sum()/len(x)) 
print (s)
item
x    100
y    300
Name: value, dtype: int64

Or divide GroupBy.sum with GroupBy.size:

g = df.groupby('item')['value']
s = g.sum() / g.size()
print (s)
item
x    100.0
y    300.0
Name: value, dtype: float64

But sum/size is mean, so solution should be simplify by GroupBy.mean:

s = df.groupby('item')['value'].mean()
print (s)
item
x    100
y    300
Name: value, dtype: int64