Series.str.zfill() behaves differently than str.zfill() from standard library

[alkamid]

Code Sample, a copy-pastable example if possible

# Pandas
>>> s = pd.Series(['-2', '+5'])
>>> s.str.zfill(5)
0    000-2
1    000+5
dtype: object

# Python 3.6
>>> '-2'.zfill(5)
'-0002'
>>> '+5'.zfill(5)
'+0005'

Problem description

People used to the standard library's str methods may expect Series.str.zfill() to behave differently, i.e. to treat '+' and '-' as special characters and move them to the left of the resulting strings if necessary. This could be a feature, not a bug, but I couldn't find any rationale behind this behaviour. This issue also affects the following methods from Series.str: pad, rjust, center.

Expected Output

# Pandas
>>> s = pd.Series(['-2', '+5'])
>>> s.str.zfill(5)
0    -0002
1    +0005
dtype: object

Output of pd.show_versions()

INSTALLED VERSIONS

commit: 563a6ad
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-39-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.23.0.dev0+807.g563a6ad
pytest: 3.5.0
pip: 9.0.3
setuptools: 39.0.1
Cython: 0.28.2
numpy: 1.14.2
scipy: 1.0.1
pyarrow: 0.9.0
xarray: 0.10.3
IPython: 6.3.1
sphinx: 1.7.3
patsy: 0.5.0
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.4
feather: 0.4.0
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: 0.8.0
psycopg2: None
jinja2: 2.10
s3fs: 0.1.4
fastparquet: 0.1.5
pandas_gbq: None
pandas_datareader: None

[TomAugspurger]

Looks like a bug. Are there any other special characters to watch out for?

[TomAugspurger]

pandas/pandas/core/strings.py

Lines 2130 to 2131 in 563a6ad

	result = str_pad(self._data, width, side='left', fillchar='0')
	return self._wrap_result(result)

is the problem area. Will need a bit of logic to handle the special characters.

[xr-chen]

Looks like this is still a bug in main, could I take this issue?

[xr-chen]

According to pandas's documentation, it seems that only zfill behaves differently from str.zfill in standard library,