Update

As Daniel rightful pointed out type(date) not str is illegal syntax. The right way to check if date is not a str:

if not isinstance(date, str):
    date = str(date)

Mathias answer is pretty good. I wrote one myself but he was faster :) Anyway here is my solution. As mentioned above precompiling the regex patterns for performance improvements is the way to go here. I would also include a pattern for the 1858.0 case which improves overall readability i think. Since python 3.6 f-strings are available and make reading code so much easier - you should start using them.

import re


PATTERNS = [
    r'(\d+)\.\d*',  # 1858.0
    r'(\d{1,2})-(\d{1,2})-(\d{4})$',  # 1-12-1963
    r'(\d{4})-(\d{1,2})-(\d{1,2})$',  # 1789-7-14
    r'(\d{4})-(\d{1,2})$',  # 1945-2
    r'(\d{1,2})-(\d{4})$'  # 2-1883
]
PATTERNS = [re.compile(p) for p in PATTERNS]


def cleanup(date):
    if type(date) not str:
        date = str(date)
    for i, p in enumerate(PATTERNS):
        res = p.match(date)
        if res and i == 0:
            return f'{res[1]}'
        if res and i == 1:
            return f'{res[3]}-{res[2] :0>2}-{res[1] :0>2}'
        if res and i == 2:
            return f'{res[1]}-{res[2] :0>2}-{res[3] :0>2}'
        if res and i == 3:
            return f'{res[1]}-{res[2] :0>2}'
        if res and i == 4:
            return f'{res[2]}-{res[1] :0>2}'


def main():
    dates = 1858.0, '1-12-1963', '1945-2', '7-2018', '1789-7-14',
    for date in dates:
        print(f'in: {date}  out: {cleanup(date)}')


if __name__ == "__main__":
    main()

Mathias answer is pretty good. I wrote one myself but he was faster :) Anyway here is my solution. As mentioned above precompiling the regex patterns for performance improvements is the way to go here. I would also include a pattern for the 1858.0 case which improves overall readability i think. Since Python 3.6 f-strings are available and make reading code so much easier - you should start using them.

import re


PATTERNS = [
    r'(\d+)\.\d*',  # 1858.0
    r'(\d{1,2})-(\d{1,2})-(\d{4})$',  # 1-12-1963
    r'(\d{4})-(\d{1,2})-(\d{1,2})$',  # 1789-7-14
    r'(\d{4})-(\d{1,2})$',  # 1945-2
    r'(\d{1,2})-(\d{4})$'  # 2-1883
]
PATTERNS = [re.compile(p) for p in PATTERNS]


def cleanup(date):
    if type(date) not str:
        date = str(date)
    for i, p in enumerate(PATTERNS):
        res = p.match(date)
        if res and i == 0:
            return f'{res[1]}'
        if res and i == 1:
            return f'{res[3]}-{res[2] :0>2}-{res[1] :0>2}'
        if res and i == 2:
            return f'{res[1]}-{res[2] :0>2}-{res[3] :0>2}'
        if res and i == 3:
            return f'{res[1]}-{res[2] :0>2}'
        if res and i == 4:
            return f'{res[2]}-{res[1] :0>2}'


def main():
    dates = 1858.0, '1-12-1963', '1945-2', '7-2018', '1789-7-14',
    for date in dates:
        print(f'in: {date}  out: {cleanup(date)}')


if __name__ == "__main__":
    main()