to_latex wrong \multicolumn count

[Phibedy]

Code Sample

import pandas as pd

pd.DataFrame({"A":[1,2,3]}).to_latex(longtable=True)

Problem description

\begin{longtable}{lr}
\toprule
{} &  A \\
\midrule
\endhead
\midrule
\multicolumn{3}{r}{{Continued on next page}} \\
\midrule
\endfoot

\bottomrule
\endlastfoot
0 &  1 \\
1 &  2 \\
2 &  3 \\
\end{longtable}

It says \multicolumn{3} but it should be 2, it only fails with longtable=True. Indeed it always writes 3 no matter what shape the table has.

Expected Output

\multicolumn{2}

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.5.2.final.0 python-bits: 64 OS: Linux OS-release: 4.10.0-35-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: None
pip: 9.0.1
setuptools: 20.7.0
Cython: None
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 6.1.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2013b
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: 0.7.3
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.8
s3fs: None
pandas_gbq: None
pandas_datareader: None

[gfyoung]

Indeed it always writes 3 no matter what shape the table has.

I'm not sure that should be the case...that being said, feel free to investigate and figure out the cause.

[Phibedy]

pandas/pandas/io/formats/format.py

Line 945 in 3c964a4

buf.write('\\multicolumn{3}{r}{{Continued on next '

line 945, the 3 is fixed, it might should be len(rows)

The testcase says also 3 line
368: \multicolumn{3}{r}{{Continued on next page}} \\

pandas/pandas/tests/io/formats/test_to_latex.py

Line 368 in 6b729dd

\multicolumn{3}{r}{{Continued on next page}} \\

If I change it the test won't work but I dont think that the test is correct.

[gfyoung]

Not sure why we are using hard-coded values. Let's change to it (both in the source code and tests) to vary, and let's see how the output looks like when we do that.

(BTW, when I say "see how the output looks", I mean that we should also make sure that it renders properly when converted to PDF for example - that would have to be done somewhat manually)

[jreback]

is this issue related: #14484

[gfyoung]

@jreback : Not as far as I can tell.