BUG: multicolumn of to_latex does not work correctly when index==False

[Alalalalaki]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

import pandas as pd
df = pd.DataFrame([[1,2,3,4]], columns=pd.MultiIndex.from_tuples([('1', 'a'), ('1', 'b'), ('2', 'a'), ('2', 'b'), ]))
print(df.to_latex(index=False, multicolumn=True,))

\begin{tabular}{rrrr}
\toprule
1 & \multicolumn{2}{l}{2} \\
a & b & a & b \\
\midrule
1 & 2 & 3 & 4 \\
\bottomrule
\end{tabular}

Issue Description

The top level of multicolumns are not located correctly.

Expected Behavior

\begin{tabular}{rrrr}
\toprule
\multicolumn{2}{l}{1} & \multicolumn{2}{l}{2} \
a & b & a & b \
\midrule
1 & 2 & 3 & 4 \
\bottomrule
\end{tabular}

Installed Versions

INSTALLED VERSIONS

commit : c7f7443
python : 3.8.10.final.0
python-bits : 64
OS : Darwin
OS-release : 20.6.0
Version : Darwin Kernel Version 20.6.0: Mon Aug 30 06:12:21 PDT 2021; root:xnu-7195.141.6~3/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : en_US.UTF-8
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.3.1
numpy : 1.21.1
pytz : 2021.1
dateutil : 2.8.2
pip : 21.2.2
setuptools : 57.4.0
Cython : 0.29.24
pytest : 6.2.4
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.6.3
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.1
IPython : 7.25.0
pandas_datareader: 0.10.0
bs4 : 4.9.3
bottleneck : None
fsspec : 2021.07.0
fastparquet : None
gcsfs : None
matplotlib : 3.4.2
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.7
pandas_gbq : None
pyarrow : 5.0.0
pyxlsb : None
s3fs : None
scipy : 1.7.0
sqlalchemy : None
tables : 3.6.1
tabulate : 0.8.9
xarray : 0.19.0
xlrd : 2.0.1
xlwt : None
numba : 0.53.1

[attack68]

Im working to phase out the pandas.LaTeXFormatter class and replace it with the Styler.template_latex jinja2 renderer instead, but this hasn't been implemented yet. (#41649)

In the meantime you can do this:

import pandas as pd
df = pd.DataFrame([[1,2,3,4]], columns=pd.MultiIndex.from_tuples([('1', 'a'), ('1', 'b'), ('2', 'a'), ('2', 'b'), ]))
styler = df.style.hide_index()
print(styler.to_latex(hrules=True, multicol_align="l")

\begin{tabular}{rrrr}
\toprule
\multicolumn{2}{l}{1} & \multicolumn{2}{l}{2} \\
a & b & a & b \\
\midrule
1 & 2 & 3 & 4 \\
\bottomrule
\end{tabular}

Please close if this address your issue.

[Alalalalaki]

Close as being solved.