groupby().agg() calls the user function for an extra time with empty inputs in 1.0.0 · Issue #31760 · pandas-dev/pandas (original) (raw)

Code Sample, a copy-pastable example if possible

df = pd.DataFrame({"key": ["a", "b", "c", "c"], "value": [1, 2, 3, 4]})

def foo(x): print(f"called with {x}") return len(x)

df.groupby("key")["value"].agg(foo)

Problem description

In 1.0.0, this code outputs:

called with Series([], Name: value, dtype: int64)
called with 0    1
Name: value, dtype: int64
called with 1    2
Name: value, dtype: int64
called with 2    3
3    4
Name: value, dtype: int64
Out[10]: 
key
a    1
b    1
c    2
Name: value, dtype: int64

In 0.25.3, the code outputs:

In [5]:  df.groupby("key")["value"].agg(foo)                                                                                                                                                                                                                                                                                                                                                                                                                                                           
called with 0    1
Name: value, dtype: int64
called with 1    2
Name: value, dtype: int64
called with 2    3
3    4
Name: value, dtype: int64
Out[5]: 
key
a    1
b    1
c    2
Name: value, dtype: int64

In 1.0.0, foo is called one more time with empty input, which can break user code

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Expected Output

Output of pd.show_versions()

[paste the output of pd.show_versions() here below this line]