pd.concat does not work correctly

Code Sample, a copy-pastable example if possible

df1.shape    # (21141, 59)
df2.shape    # (21141, 6)
result = pd.concat([df1, df2], axis=1, ignore_index=True)
result.shape    # (42282, 65)

Problem description

I have 2 dataframes that I try to concatenate horizontally. The method concat doesn’t work: it returns a dataframe with a wrong dimension. Moreover, all column names happen to be changed to numbers going from 0 to 64…

The dataframes are created from a dataset that is a bit big so I cannot reproduce the creation code here but I can provide you with more details by e-mail.

Expected Output

The right dimension should be (21141, 65) and the resulting columns should be just the concatenation of df1’s columns and df2’s columns.

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.5.4.final.0 python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel byteorder: little LC_ALL: None LANG: fr LOCALE: None.None

pandas: 0.22.0 pytest: 3.3.2 pip: 18.0 setuptools: 39.0.1 Cython: 0.27.3 numpy: 1.14.2 scipy: 1.0.0 pyarrow: None xarray: None IPython: 6.2.1 sphinx: 1.6.6 patsy: 0.5.0 dateutil: 2.6.1 pytz: 2017.3 blosc: None bottleneck: 1.2.1 tables: 3.4.2 numexpr: 2.6.4 feather: None matplotlib: 2.1.2 openpyxl: 2.4.10 xlrd: 1.1.0 xlwt: 1.3.0 xlsxwriter: 1.0.2 lxml: 4.1.1 bs4: 4.6.0 html5lib: 1.0.1 sqlalchemy: 1.2.1 pymysql: None psycopg2: None jinja2: 2.10 s3fs: None fastparquet: None pandas_gbq: None pandas_datareader: None

[TomAugspurger]

It’s hard to say without a minimal example, but it appears that you’re getting confused by the alignment. See http://pandas-docs.github.io/pandas-docs-travis/reference/api/pandas.concat.html?highlight=concat#pandas.concat

Specifically, the ignore_index_parameter

Note the index values on the other axes are still respected in the join.

Since you’re using axis=1,

1. the column labels will be reset to [0, n)
2. the row labels will be preserved (and aligned).

If you really don’t care about your row labels, then you’ll want to drop the row labels before concating pd.concat([df1.reset_index(drop=True), df2.reset_index(drop=True)], ...)

[mfouesneau]

I agree with @Mark531 there should be an intuitive manner to merge dataframes horizontally. the documentation on ignore_index=True is unclear, I also spent time on this.