negative XIRR is not calculated

Hi, I was testing your code for XIRR. But few times when I was trying to get negative XIRR it raised error (python 2.7). Example data:

[(datetime.date(2014, 2, 27), -4000.0), (datetime.date(2015, 3, 6), 2050.2)]

Error:

Traceback (most recent call last):
  File "./test.py", line 17, in <module>
    calculate_xiir_by_portfolio(Portfolio.objects.get(pk=23))
  File "/var/www/django/digrin27/wsgi/digrin/portfolio/utils.py", line 227, in calculate_xiir_by_portfolio
    print xirr(cashflows)
  File "/var/www/django/digrin27/wsgi/digrin/lib/financial.py", line 78, in xirr
    return optimize.newton(lambda r: xnpv(r,cashflows),guess)
  File "/usr/lib/python2.7/dist-packages/scipy/optimize/zeros.py", line 159, in newton
    q1 = func(*((p1,) + args))
  File "/var/www/django/digrin27/wsgi/digrin/lib/financial.py", line 78, in <lambda>
    return optimize.newton(lambda r: xnpv(r,cashflows),guess)
  File "/var/www/django/digrin27/wsgi/digrin/lib/financial.py", line 54, in xnpv
    return sum([cf/(1+rate)**((t-t0).days/365.0) for (t,cf) in chron_order])
ValueError: negative number cannot be raised to a fractional power

Not sure about excell, but google spreadsheet calculates XIRR = -0.4809631525. LibreOffice returns error 502. Is it normal behaviour and your code works properly? What is the issue and what would it take to get same results as google docs (support more input values)?

peliot / XIRR-and-XNPV

negative XIRR is not calculated #4