數字加上千分符號

I got this to work:

>>> import locale
>>> locale.setlocale(locale.LC_ALL, 'en_US')
'en_US'
>>> locale.format("%d", 1255000, grouping=True)
'1,255,000'

Sure, you don't need internationalization support, but it's clear, concise, and uses a built-in library.

P.S. That "%d" is the usual %-style formatter. You can have only one formatter, but it can be whatever you need in terms of field width and precision settings.

P.P.S. If you can't get locale to work, I'd suggest a modified version of Mark's answer:

def intWithCommas(x):
    if type(x) not in [type(0), type(0L)]:
        raise TypeError("Parameter must be an integer.")
    if x < 0:
        return '-' + intWithCommas(-x)
    result = ''
    while x >= 1000:
        x, r = divmod(x, 1000)
        result = ",%03d%s" % (r, result)
    return "%d%s" % (x, result)

Recursion is useful for the negative case, but one recursion per comma seems a bit excessive to me.

---

I'm surprised that no one has mentioned that you can do this with f-strings in Python 3.6 as easy as this:

>>> num = 10000000
>>> print(f"{num:,}")
10,000,000

* where the part after the colon is the format specifier. The comma is the separator character you want, so f"{num:_}" uses underscores instead of a comma.

• This is equivalent of using format(num, ",") for older versions of python 3.

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