问题
I'm new to pandas and have a very basic question, please!
On Python v3.6 through spyder:
x= pd.DataFrame(columns = ['1','2'])
print(x)
x['1'] = '25'
print(x)
From the print statements, the dataframe x does not appear to change.
My question: What does x['1'] = '25' do, if anything?
回答1:
There is actually a difference between the semantics of assigning scalars and iterables (think containers such as lists as list-like objects).
Consider,
df = pd.DataFrame(columns=['1', '2'])
df
Empty DataFrame
Columns: [1, 2]
Index: []
You've defined an empty dataframe without any index (no rows), but only a schema for the columns.
When you assign a scalar to a column, the assignment is broadcast across all rows. In this case, since there are none, nothing happens:
df['1'] = 123
df
Empty DataFrame
Columns: [1, 2]
Index: []
However, assigning a list-like iterable is a different story, as pandas will create new rows for it:
df['1'] = [123]
df
1 2
0 123 NaN
Now, to understand how scalar assignment works, consider a similar empty DataFrame, but with a defined index:
df = pd.DataFrame(columns=['1', '2'], index=[0, 1])
df
1 2
0 NaN NaN
1 NaN NaN
it is still "empty" (not really), but now we can assign scalars and the assignment is broadcast,
df['1'] = 123
df
1 2
0 123 NaN
1 123 NaN
Contrast this behaviour with that previously shown.
来源：https://stackoverflow.com/questions/56602703/assigning-a-scalar-value-to-an-empty-dataframe-doesnt-appear-to-do-anything