Posts tagged python

Fun With Numbers

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Periodically, I try to take a look at our home finances to see if there’s something that can be done to find some hidden stash of money. So far, my efforts have been for naught.

One expense I always investigate is our mortgage payment. I’ve always tried to pay ahead on the mortgage to save on future interest payments. So yesterday I got curious about what the best way to pay the curtailment- at the same time as the payment or halfway through the month or some other day of the month? I could have resorted to a web page that calculates amortization tables, but what fun is that?

So I wrote some python code that can be used to generate a repayment table.

Here’s the meat of it:

Months = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31] 
Month = 0

def setPaymentParameters(payment, rate, day = 0):

    monthlyrate = rate / 100.0 / 12

    def _calcMonth(principal, curtailment = 0):

        def _calcADB(principal, curtailment, day):
            global Month, Months

            dim = Months[Month]
            Month += 1
            if Month == 12:
                Month = 0
            return ((day * (principal + curtailment)) + ((dim - day) * principal))/dim

        # _calcMonth code starts here...
        adb = _calcADB(principal, curtailment, day)
        interest = adb * monthlyrate
        return (principal,
                adb,
                interest, 
                payment-interest,  # principal payment
                curtailment,
                (payment-interest)+curtailment,  # total principal payment
                principal-((payment-interest)+curtailment))

    return _calcMonth

So the setPaymentParameters function returns a function that will calculate the monthly interest, principal payment and so forth for a single month. The function returned is a closure over the set monthly payment, the interest rate and the day of month a theoretical curtailment payment is made. No curtailment is necessary for the function to work.

In order to determine the effect of curtailments separate from the normal payment, the calculation uses an average daily balance method. For instance, a normal payment is typically made on the 1st of the month and a separate curtailment payment is made on the 15th. The average is calculated by summing the days the principal the post-payment level and adding the sum of the days the principal is at the post-curtailment level. Then divide by the total number of days to get the average daily balance. In the absence of a curtailment, the calculation simplifies to the prinicipal balance at the beginning of the period.

Following is an example of how to use the function:

Rate = float(5.25)
Payment = float(1000.00)

Amortization = []
CalcMonth = setPaymentParameters(Payment, Rate, 15)
Principal = float(150000.00)
while (Principal > 0):
    t = CalcMonth(Principal)
    Amortization.append(t)
    Principal = t[6]

print len(Amortization)
interestTotal = float(0.0)
for i in range(len(Amortization)):
    print map(lambda x: format(x, ".2f"), Amortization[i])
    interestTotal += Amortization[i][2]

print format(interestTotal, ".2f")

The output won’t be particularly pretty, but it will list the total number of payments made to payoff the loan, followed by a breakdown of the effect of each monthly payment, followed by a calculation of the total interest paid. A monthly payment line will look like this:

['150000.00', '150000.00', '656.25', '343.75', '0.00', '343.75', '149656.25']

From left to right, we have the beginning principal, the average daily balance, the interest for the month, the principal paydown, the principal curtailment, the total principal paydown and finally the principal balance after the payment is applied. Each subsequent month uses this final principal balance number as the beginning balance.

The above snippet doesn’t use a curtailment payment to accelerate the paydown of the mortgage. To do that, the while loop needs to be modified slightly:

Curtailment = float(500.00)
while (Principal > 0):
    if len(Amortization) == 0:
        temp = CalcMonth(Principal)
        t = (temp[0],
             temp[1], 
             temp[2],
             temp[3], 
             Curtailment, 
             Payment-temp[2]+Curtailment,
             Principal-(Payment-temp[2]+Curtailment))
    else:
        t = CalcMonth(Principal, Curtailment)
    Amortization.append(t)
    Principal = t[6]

The modification is needed for the first payment. Since it’s the first payment, no curtailment is made, so the interest is calculated on the entire loan amount. The returned payment info needs to be modified then, manually inserting the curtailment payment. Thereafter, all calculations use the curtailment.

Here are the first couple of payment output lines:

['150000.00', '150000.00', '656.25', '343.75', '500.00', '843.75', '149156.25']
['149156.25', '149424.11', '653.73', '346.27', '500.00', '846.27', '148309.98']

The curtailment payment is included and the ending principal balance includes the extra payment. Notice the second line’s average daily balance number, which is higher than the starting principal balance. To fully understand that, first notice that the setPaymentParameters was called with the day set to 15, meaning the curtailment payment is applied on the 15th of the month, not the same day as the normal payment. Therefore, there are 15 days where the principal sits without the curtailment payment applied. Then the payment is applied for the remainder of the month. The end result is the ADB, which is used to calculate interest, is slightly higher than the principal balance after the curtailment.

The final answer to my question about the optimal day to apply the curtailment turned out to be- it saves the most money if the curtailment is paid on the same day as the normal payment. This makes sense since in general, paying earlier means the outstanding principal is reduced quicker, therefore interest is minimized.

But, that’s not the whole picture. Sometimes, for monthly household cash flow purposes, it is preferable to make multiple smaller payments. Will that result in a big difference in total interest paid? The answer there turns out to be no, it won’t. Depending on the amount owed and repayment length, the difference is only a few hundred dollars.

Release Announce- blogtool v1.1.0

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I’ve just uploaded blogtool v1.1.0 to pypi.

The minor release number bump is due to switching the option parser library as well as adding the ability to process information from the standard input. The comment option has also been modified to take a couple of arguments.

I’ve added some spiffy, new web based documentation to help with getting up and running with blogtool. The documentation stuff was generated with the help of sphinx, a very cool tool that uses a different plain-text markup format that I’ll be exploring adding support for in blogtool.

Announce- blogtool v1.0.1

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I’ve released blogtool version 1.0.1 into the wild.

This is a bug fix version. It fixes an error in HTML output where tags like \<img> were not being properly closed. Also takes care of stray ‘&’ characters that need to be escaped.

It also fixes some bugs in the getpost option related to converting the post HTML into it’s markdown equivalent. Nested inline elements were not properly accounted for and escaping of a number of characters was also added.

Release Announcement- blogtool

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I wrote a blog client a couple years ago and have been developing it on and off ever since. One of the reasons I hadn’t done anything public with it is I needed to take the time to organize it appropriately for something like pypi.

I’ve finally taken those steps and have put it out into the wild. The source code is on github, here. I’ve also used python’s setuptools to publish it on pypi, here.

It works with my self-hosted WordPress blog and I’ve used it almost for all but a handful of the blog posts I’ve written on the blog, so I consider it reasonably well tested for those purposes. It won’t support all of WordPress features, but I plan on changing that as I migrate some of the functionality over to using more of the WordPress API. When I originally wrote blogtool, WordPress didn’t have its own API for posting, so that’s why that shortcoming exists.

There are a couple of nice features to blogtool that I thought I’d mention here. One, it uses python-markdown to mark-up post text. It’s proven very capable for my style of blogging, which is 90% text. It handles pictures as well, and I’ve added a little wrinkle for that purpose. Rather than supply a URL or some such for markdown's syntax, simply supply a file path to the picture. Then, blogtool will take care of the rest.

The other nice feature is that posts can be retrieved and edited from a blog. When retrieving, it will reformat the HTML into markdown style format. This is useful for editing comments as well as posts.

So, there it is. My first published code project.

python-markdown Typed-List Extension

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I’ve contributed a bit to the python-markdown project in the form of bug fixes. Today I finished creating an extension that allows python-markdown to recognize list types and generate code with the appropriate list markers. For instance, lists can be marked with upper or lower case letters, or upper or lower case Roman numerals.

The git repository for the extension is available here.

Down and Dirty Mail Notification

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Following is a simple new mail notification implementation for the awesome window manager that leverages procmail. It’s main virtue is simplicity: there are about 20 lines of python code, 1 procmail recipe and several lines of code required in the rc file for awesome. The result is a numeric count of new email displayed in the statusbar.

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uzbl and dmenu

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I’ve been playing around with uzbl again and decided it was high time I tried out dmenu. What I’d read made it sound pretty slick, I was just leery of having to learn how to work with another application. Thankfully, dmenu is extremely easy to use. It isn’t available as a deb package, but the source is readily available. I built it with the vertical patch.

As practice I figured I’d rewrite one of the stock uzbl scripts as a python script. I chose the load_url_from_bookmarks.sh script since it was pretty easy for me to decipher. That’s not sayin’ much since my bash scripting foo is, well- ‘miserable’ is probably the right word.

The exercise proved valuable for a couple of reasons.

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More uzbl

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I’ve managed to get uzbl and awesome to talk with one another. Without using awesome-client and uzblctrl or socat. It turned out, upon further examination, that awesome-client is just a bash script for communicating on dbus and uzblctrl is just a simple c program for communicating over a socket.

These findings suggested that if I could get Lua to talk over a socket and if I could come up with a python script to talk over dbus, I would have a more flexible means for getting uzbl and awesome to talk with one another. Luckily for me, python has a dbus module and lua has luasockets.

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