Pandas concat yields ValueError: Plan shapes are not aligned

Question

In pandas, I am attempting to concatenate a set of dataframes and I am getting this error:

ValueError: Plan shapes are not aligned

My underst

Answer 1

How to reproduce above error from pandas.concat(...):

ValueError: Plan shapes are not aligned

The Python (3.6.8) code:

import pandas as pd
df = pd.DataFrame({"foo": [3] })
print(df)
df2 = pd.concat([df, df], axis="columns")
print(df2)
df3 = pd.concat([df2, df], sort=False) #ValueError: Plan shapes are not aligned

which prints:

   foo
0    3

   foo  foo
0    3    3
ValueError: Plan shapes are not aligned

Explanation of error

If the first pandas dataframe (here df2) has a duplicate named column and is sent to pd.concat and the second dataframe isn't of the same dimension as the first, then you get this error.

Solution

Make sure there are no duplicate named columns:

df_onefoo = pd.DataFrame({"foo": [3] })
print(df_onefoo)
df_onebar = pd.DataFrame({"bar": [3] })
print(df_onebar)
df2 = pd.concat([df_onefoo, df_onebar], axis="columns")
print(df2)
df3 = pd.concat([df2, df_onefoo], sort=False)
print(df2)

prints:

   foo
0    3

   bar
0    3

   foo  bar
0    3    3

   foo  bar
0    3    3

Pandas concat could have been more helpful with that error message. It's a straight up bubbleup-implementation-itis, which is textbook python.