#### Code Sample, a copy-pastable example if possible The following code ```python import pandas as pd dfs_sq = [] dfs_sq.append(pd.DataFrame(index=[0, 'sess'], columns=range(1,3))) for i in range(5): dfs_sq.append(pd.DataFrame(index=[0, 1, 'sess'], columns=range(1,3))) df_sq = pd.concat(dfs_sq, axis=1) assert df_sq.index[1] == 'sess', df_sq.index ``` ... saved as ``dependable.py``, succeeds if called as ``PYTHONHASHSEED=40 python3 dependable.py`` and fails if called as ``PYTHONHASHSEED=41 python3 dependable.py``: ```bash Traceback (most recent call last): File "dependable.py", line 11, in <module> assert df_sq.index[1] == 'sess', df_sq.index AssertionError: Index([0, 1, 'sess'], dtype='object') ``` #### Problem description This is not very nice. #### Expected Output The same - which one is not particularly important. #### Output of ``pd.show_versions()`` <details> INSTALLED VERSIONS ------------------ commit: None python: 3.5.3.final.0 python-bits: 64 OS: Linux OS-release: 4.9.0-3-amd64 machine: x86_64 processor: byteorder: little LC_ALL: None LANG: it_IT.UTF-8 LOCALE: it_IT.UTF-8 pandas: 0.21.0.dev+389.g276f3089a pytest: 3.0.6 pip: 9.0.1 setuptools: None Cython: 0.25.2 numpy: 1.12.1 scipy: 0.19.0 pyarrow: None xarray: None IPython: 5.1.0.dev sphinx: 1.5.6 patsy: 0.4.1 dateutil: 2.6.0 pytz: 2017.2 blosc: None bottleneck: 1.2.1 tables: 3.3.0 numexpr: 2.6.1 feather: 0.3.1 matplotlib: 2.0.2 openpyxl: None xlrd: 1.0.0 xlwt: 1.1.2 xlsxwriter: 0.9.6 lxml: None bs4: 4.5.3 html5lib: 0.999999999 sqlalchemy: 1.0.15 pymysql: None psycopg2: None jinja2: 2.9.6 s3fs: None fastparquet: None pandas_gbq: None pandas_datareader: 0.2.1 </details>