DOC/ERR: better error message on no common merge keys · Issue #19391 · pandas-dev/pandas (original) (raw)

Currently, merging two DataFrames on indices is not a default option. When the keyword argument on is not specified, MergeError is occurred.

Code Sample and Problem Statement

In [1]: import pandas as pd In [2]: a = pd.DataFrame({'a':[1,2]}) In [3]: b = pd.DataFrame({'b':[10,20]}) In [4]: a.merge(b)

MergeError Traceback (most recent call last) in () ----> 1 a.merge(b)

/home/swyoon/env/swyoon/local/lib/python2.7/site-packages/pandas/core/frame.pyc in merge(self, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy, indicator, validate) 5368 right_on=right_on, left_index=left_index, 5369 right_index=right_index, sort=sort, suffixes=suffixes, -> 5370 copy=copy, indicator=indicator, validate=validate) 5371 5372 def round(self, decimals=0, *args, **kwargs):

/home/swyoon/env/swyoon/local/lib/python2.7/site-packages/pandas/core/reshape/merge.pyc in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy, indicator, validate) 55 right_index=right_index, sort=sort, suffixes=suffixes, 56 copy=copy, indicator=indicator, ---> 57 validate=validate) 58 return op.get_result() 59

/home/swyoon/env/swyoon/local/lib/python2.7/site-packages/pandas/core/reshape/merge.pyc in init(self, left, right, how, on, left_on, right_on, axis, left_index, right_index, sort, suffixes, copy, indicator, validate) 558 warnings.warn(msg, UserWarning) 559 --> 560 self._validate_specification() 561 562 # note this function has side effects

/home/swyoon/env/swyoon/local/lib/python2.7/site-packages/pandas/core/reshape/merge.pyc in _validate_specification(self) 951 self.right.columns) 952 if len(common_cols) == 0: --> 953 raise MergeError('No common columns to perform merge on') 954 if not common_cols.is_unique: 955 raise MergeError("Data columns not unique: {common!r}"

MergeError: No common columns to perform merge on

For a successful merge, we need to specify rather verbose keyword arguments left_index=True, right_index=True.

In [5]: a.merge(b, left_index=True, right_index=True) Out[5]: a b 0 1 10 1 2 20

Expected Output

What if we could do

In [1]: import pandas as pd In [2]: a = pd.DataFrame({'a':[1,2]}) In [3]: b = pd.DataFrame({'b':[10,20]}) In [4]: a.merge(b) Out[4]: a b 0 1 10 1 2 20

A briefer syntax for merge on index could facilitate the usability.

Output of pd.show_versions()

In [5]: pd.show_versions()

INSTALLED VERSIONS

commit: None python: 2.7.12.final.0 python-bits: 64 OS: Linux OS-release: 4.2.0-42-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: en_US.UTF-8 LANG: en_US.UTF-8 LOCALE: None.None

pandas: 0.22.0 pytest: 3.1.3 pip: 9.0.1 setuptools: 38.2.4 Cython: None numpy: 1.14.0 scipy: 0.19.1 pyarrow: None xarray: None IPython: 5.4.1 sphinx: None patsy: 0.4.1 dateutil: 2.6.1 pytz: 2017.3 blosc: None bottleneck: None tables: None numexpr: None feather: None matplotlib: 2.0.2 openpyxl: 2.4.8 xlrd: 1.1.0 xlwt: None xlsxwriter: None lxml: 3.7.3 bs4: 4.6.0 html5lib: 0.9999999 sqlalchemy: 1.1.15 pymysql: None psycopg2: 2.7.1 (dt dec pq3 ext lo64) jinja2: 2.9.6 s3fs: None fastparquet: None pandas_gbq: None pandas_datareader: None