BUG: __rmatmul__ error handling

[minggli]

Code Sample, a copy-pastable example if possible

when dataframe.__rmatmul__ is triggered, the DataFrame.dot is called. However, the error message doesn't seem to align with numpy.

import numpy as np
from pandas import *

a = np.random.rand(10, 4)
b = np.random.rand(5, 3)

df = DataFrame(b)
# a.__matmul__ is called
a @ df

# df.__rmatmul__ is called
a.tolist() @ df

Problem description

>>> # a.__matmul__ is called
... a @ df
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ValueError: shapes (10,4) and (5,3) not aligned: 4 (dim 1) != 5 (dim 0)
>>>
>>> # df.__rmatmul__ is called
... a.tolist() @ df
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/Users/mingli/GitHub/open/pandas/pandas/core/frame.py", line 901, in __rmatmul__
    return self.T.dot(np.transpose(other)).T
  File "/Users/mingli/GitHub/open/pandas/pandas/core/frame.py", line 879, in dot
    r=rvals.shape))
ValueError: Dot product shape mismatch, (3, 5) vs (4, 10)

Expected Output

ValueError: Dot product shape mismatch, (10, 4) vs (5, 3)

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: 7a74af7 python: 3.5.5.final.0 python-bits: 64 OS: Darwin OS-release: 17.4.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8

pandas: 0.24.0.dev0+147.g7a74af7ec
pytest: 3.6.1
pip: 10.0.1
setuptools: 28.8.0
Cython: 0.28.3
numpy: 1.14.5
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: 1.7.5
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[gfyoung]

xref #21530 #21578 (blocked by this it seems based on #21578 (comment))

cc @jreback