How to add columns to an empty pandas dataframe?

Question

I have an empty dataframe.

df=pd.DataFrame(columns=['a'])

for some reason I want to generate df2, another empty dataframe, with two columns ‘a’ and ‘b’.

If I do

df.columns=df.columns+'b'

it does not work (I get the columns renamed to ‘ab’)
and neither does the following

df.columns=df.columns.tolist()+['b']

How to add a separate column ‘b’ to df, and df.emtpy keep on being True?

Using .loc is also not possible

   df.loc[:,'b']=None

as it returns

  Cannot set dataframe with no defined index and a scalar

Asked By: 00__00__00

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Source

Answer 1

If you just do df['b'] = None then df.empty is still True and df is:

Empty DataFrame
Columns: [a, b]
Index: []

EDIT:
To create an empty df2 from the columns of df and adding new columns, you can do:

df2 = pd.DataFrame(columns = df.columns.tolist() + ['b', 'c', 'd'])

Answered By: Ben.T

Answer 2

This is one way:

df2 = df.join(pd.DataFrame(columns=['b']))

The advantage of this method is you can add an arbitrary number of columns without explicit loops.

In addition, this satisfies your requirement of df.empty evaluating to True if no data exists.

Answered By: jpp

Answer 3

If you want to add multiple columns at the same time you can also reindex.

new_cols = ['c', 'd', 'e', 'f', 'g']
df2 = df.reindex(df.columns.union(new_cols), axis=1)

#Empty DataFrame
#Columns: [a, c, d, e, f, g]
#Index: []

Answered By: ALollz

Answer 4

Here are few ways to add an empty column to an empty dataframe:

df=pd.DataFrame(columns=['a'])
df['b'] = None
df = df.assign(c=None)
df = df.assign(d=df['a'])
df['e'] = pd.Series(index=df.index)   
df = pd.concat([df,pd.DataFrame(columns=list('f'))])
print(df)

Output:

Empty DataFrame
Columns: [a, b, c, d, e, f]
Index: []

I hope it helps.

Answered By: Sumit Jha

Answer 5

You can use concat:

df=pd.DataFrame(columns=['a'])
df
Out[568]: 
Empty DataFrame
Columns: [a]
Index: []

df2=pd.DataFrame(columns=['b', 'c', 'd'])
pd.concat([df,df2])
Out[571]: 
Empty DataFrame
Columns: [a, b, c, d]
Index: []

Answered By: BENY

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