8000 BUG: removed unnecesary warning from financial.pmt method by githubmlai · Pull Request #6073 · numpy/numpy · GitHub
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BUG: removed unnecesary warning from financial.pmt method #6073

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BUG: removed unnecesary warning from financial.pmt method
githubmlai Jul 13, 2015
05587f9
BUG: removed unnecesary warning from financial.pmt method
githubmlai Jul 13, 2015
1b76a7e
Merge branch 'issue5046' of github.com:githubmlai/numpy into issue5046
githubmlai Jul 14, 2015
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BUG: removed unnecesary warning from financial.pmt method 
Previous implementation would cause divide by zero and hence warning since
"fact" variable was calculated for all rates (both zero and non-zero) but then
zero rates would be masked with a where equality.

Changed implementation to apply non-zero rates using "np.divide( numerator,
denominator, where=mask_of_zero_values)"and then apply zero rates with "np.copyto".

Also updated documentation to show exact formulas that are being calculated.

Done with help of Sebastian Berg in Austin,TX!

7/13/15 Added in parenthesis for reability per Nathaniel Smith comment e.g.
    zero_rate_mask = (rate == 0)

Fixes issue #5046
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githubmlai committed Jul 13, 2015
commit 05587f9f071408084ca643d81e9bfa1d112303b8
30 changes: 20 additions & 10 deletions numpy/lib/financial.py
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Original file line number Diff line number Diff line change
Expand Up @@ -148,7 +148,7 @@ def pmt(rate, nper, pv, fv=0, when='end'):
Number of compounding periods
pv : array_like
Present value
fv : array_like, optional
fv : array_like (optional)
Future value (default = 0)
when : {{'begin', 1}, {'end', 0}}, {string, int}
When payments are due ('begin' (1) or 'end' (0))
Expand All @@ -163,17 +163,24 @@ def pmt(rate, nper, pv, fv=0, when='end'):

Notes
-----
The payment is computed by solving the equation::
When `rate != 0``: the payment is computed by solving the equation::

fv +
pv*(1 + rate)**nper +
pmt*(1 + rate*when)/rate*((1 + rate)**nper - 1) == 0

or, when ``rate == 0``::
Solving for pmt yields::

pmt = -(fv + pv*(1 + rate)**nper) * rate / ( (1 + rate*when) * ((1 + rate)**nper -1) )

When ``rate == 0``:the payment is computed by instead solving the equation::

fv + pv + pmt * nper == 0

for ``pmt``.
Solving for pmt yields::

pmt = -(fv + pv) / nper


Note that computing a monthly mortgage payment is only
one use for this function. For example, pmt returns the
Expand Down Expand Up @@ -208,12 +215,15 @@ def pmt(rate, nper, pv, fv=0, when='end'):
"""
when = _convert_when(when)
(rate, nper, pv, fv, when) = map(np.asarray, [rate, nper, pv, fv, when])
temp = (1 + rate)**nper
mask = (rate == 0.0)
np.copyto(rate, 1.0, where=mask)
z = np.zeros(np.broadcast(rate, nper, pv, fv, when).shape)
fact = np.where(mask != z, nper + z, (1 + rate*when)*(temp - 1)/rate + z)
return -(fv + pv*temp) / fact

# use different payment if rate == 0
zero_rate_mask = (rate == 0)
compounded_rate = (1 + rate)**nper
payment = np.asarray(np.divide(-(fv + pv*compounded_rate)*rate,
(1 + rate*when)*(compounded_rate - 1),
where=~zero_rate_mask))
np.copyto(payment, -(fv + pv)/nper, where=zero_rate_mask)
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(Sorry, got interrupted half-way through reviewing :-))
I'd put spaces around most or all of these binary operators (e.g. (1 + rate * when) * (compounded_rate - 1)), though in this case it's more a matter of personal taste than rigid policy. (In general spaces around operators is PEP8's recommendation, but there's an escape clause for higher-priority operators that could apply here if one wanted.)

return payment

def nper(rate, pmt, pv, fv=0, when='end'):
"""
Expand Down
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