T-Th 9:05
or
T-Th 11:15
in Olin 155

CS 1110: Introduction to Computing Using Python

Spring 2014

Assignment 1:
Currency

Due to CMS by Tuesday, February 25th at 11:59 pm.

Thinking about that trip overseas? It is best to go when the exchange rate is in your favor, if you can swing it. When your dollars buy more in the foreign currency, you can do more on your vacation. Hence, it would be nice to have a function that, given your current amount of cash in US dollars, tells you how much your money is worth in another currency.

However, there is no set mathematical formula to compute this conversion. Rather, the value of one currency with respect to another is constantly changing. In fact, in the time that it takes you to read this paragraph, the exchange rate between the dollar and the Euro has probably changed several times. How on Earth do we write a program to handle something like that?

One solution is to make use of a web service. A web service is a program that, when you send it web requests, automatically generates a web page with the information that you asked for. In our case, the web service will tell us the current exchange rate for most of the major international currencies. Your job will be to use string-manipulation methods to read the web page and extract the exact information we need. Full instructions are included below.

Learning Objectives

This assignment is designed to give you practice with the following skills:

• How to write a self-contained module in Python
• How to use string and object methods in Python
• How to connect Python to a web service
• How to read specifications and understand preconditions
• How to use docstrings appropriately for specifications
• How to follow the coding conventions for this course
• How to thoroughly test a program

The functions we ask you to write in this assignment are relatively short and straightforward. The emphasis is testing and "good practices", not complicated computations. You will find the most recent lab very helpful in understanding this assignment.

Table of Contents

Authors: W. White, D. Yoon, Q. Jia, L. Lee, and S. Marschner. Currency calculator code by N. Gavalas.

Image Credit: Petr Kratochvil

2. Academic Integrity and Collaboration
3. Before You Get Started
4. The Currency Exchange Web Service
5. Iterative Development
6. Part A: Breaking up Strings
7. Part B: Processing a JSON String
8. Part C: Currency Query
9. Part D: Currency Exchange
10. Finishing the Assignment
11. Appendix: Connecting to Rate-Exchange Service

Academic Integrity and Collaboration

This assignment is a slightly modified version of an assignment given the previous semester. Please do this assignment without consulting (or seeking) previous solutions. Since you are allowed to revise and resubmit, with help from us, until you have mastered this assignment, there is no reason to not do this assignment on your own. Furthermore, consulting any prior solution is a violation of CS1110's academic integrity policies.

Conversely, you should not enable violations of academic policy either. Do not post your code to Pastebin, GitHub, or any other publicly accessible site. This is also a violation of academic integrity.

Collaboration Policy

You may do this assignment with one other person. If you are going to work together, then form your group on CMS as soon as possible. This must be completed before you submit the assignment. Both people must do something to form the group. The first person proposes, and then the other accepts. You have to do this early because CMS does not allow you to form groups once grades are released. Once you've grouped on CMS, only one person submits the files.

If you do this assignment with another person, you must work together. It is against the rules for one person to do some programming on this assignment without the other person sitting nearby and helping. Take turns "driving"; alternate using the keyboard and mouse.

With the exception of your CMS-registered partner, you may not look at anyone else's code or show your code to anyone else (except a CS1110 staff member) in any form whatsoever.

Before You Get Started

Read Carefully

These instructions may seem long, but that is because we have tried to give you all the information you need in one document. Your chances of completing the assignment quickly will be increased by reading carefully and following all instructions. Many requests for resubmission are caused not by issues with programming but simply by not following instructions.

Pay particular attention to the section on Iterative Development, as it contains important instructions for the remaining sections, and we will not repeat these instructions for each section.

Start Early!

500 students trying to contact a web service at once will slow everybody down. Connecting to, and reading from, a web page is not instantaneous. It will take several seconds for some of the functions you will write to complete. Furthermore, if you wait until the last minute to test this assignment, you will be connecting to the same web page as everyone else in the class, so things could slow down even more.

Grading Policy (Revise-and-Resubmit Cycle)

To ensure that everyone masters this assignment, we will use an iterative feedback process. If one of the objectives below is not properly met, we will give you feedback and expect you to revise and resubmit. This process will continue until you are done. This process should be finished by Sunday, March 2nd; Once you finish you will receive a perfect score of 10. In our experience, almost everyone is able to achieve a perfect score within two submissions.

In grading your code, we will focus on the following issues in order:

• Correct function specifications and/or formatting
• Adequate test cases
• Correctness of the code (does it pass our test cases?)

Formatting is graded according to the course style guidelines, available on the course web page.

If your code fails one of the three tests above, we will notify you and ask you to submit. We stop checking once we find the first few errors, so you should not assume that the errors we point out are the only errors present.

Until we have decided that you have mastered (e.g. 10/10) the assignment, your "grade" on CMS will be the number of revisions so far. This allows us to keep track of your progress. Do not be alarmed if you see a "1" for the assignment at first! The assignment will be considered completed when it passes all three steps outlined above.

Assignment Scope

Everything that you need to complete this assignment should have been covered by Lecture 6 (Function Calls) in class. In particular, you may not use if-statements anywhere in this assignment, as they are not necessary. Submissions containing if-statements will be returned for you to revise. Similarly, students with prior programming experience should not try to use loops or recursion.

Getting Help

If you do not know where to start, if you do not understand testing, or if you are completely lost, please see someone immediately. This can be the course instructor, a TA, or a consultant. Do not wait until the last minute, particularly since this is due just after a weekend. A little in-person help can do wonders. See the staff page for more information.

The Currency Exchange Web Service

Before you do anything at all, you might want to play around with the currency exchange web service. You do not need any Python to do this; just a web browser.

For this assignment, you will use a simulated currency exchange service that never changes values. This is important for testing; if the answer is always changing, it is hard to test that you are getting the right answers. The appendix explains how you can make a few minor changes to hook your program up to Rate-Exchange currency calculator, to get real-time currency-exchange results; however, do not submit such code for your assignment.

To use the service, you employ special URLs that start with the following prefix:

  http://cs1110.cs.cornell.edu/2014sp/a1/calculator.php?

This prefix is followed with a currency query. A currency query has three pieces of information in the following format (without spaces; we have included spaces here solely for readability):

  from=source & to=target & q=amount

where source is a three-letter code for the original currency, target is a three-letter code for the new currency and amount is a float value for the amount of money in the original. For example, if you want to know the value of 2.5 dollars (USD) in Euros (EUR), the query is

  from=USD&to=EUR&q=2.5

The full URL for this query is

http://cs1110.cs.cornell.edu/2014sp/a1/calculator.php?from=USD&to=EUR&q=2.5

Click on the link to see it in action.

You will note that the "web page" in your browser is just a single line in the following format:

  {"to": "EUR", "rate": 0.75443, "from": "USD", "v": 1.88608072500000}

This is what is known as a JSON representation of the answer. JSON is a way of encoding complex data. You will use what you know about string operations and methods to pull out the relevant data out of the JSON string.

You should try a few more currency queries to familiarize yourself with the service. Note that if you enter an invalid query (for example, using a non-existent currency code like "AAA"), you will get the following response in error:

  {"err": "failed to parse response from xe.com."} 

Let's try something more extreme this time. If you enter a query without any parameters passed, you will get this:

  {"err": "invalid request"} 

This will be important for error handling in this assignment.

Your query can be partially valid, but still have unorthodox parts. In this case, the server responds with a warning. For example, if you supply a query with two valid currency codes, but with an invalid quantity value, you will get the following warning:

  {"to": "EUR", "rate": 0.75443, "warning": "invalid quantity, ignored.", "from": "USD"} 

But for this assignment, you can ignore these warning cases. We focus on completely erroneous queries or perfectly well-formed queries only only.

Focus of the Assignment

Your primary goal in this assignment is to use the currency exchange service to write the following function:

def exchange(amount_from, currency_from, currency_to):
    """Returns: amount of currency received in the given exchange.

    In this exchange, the user is changing amount_from money in
    currency currency_from to the currency currency_to.
    The value returned represents the amount in currency currency_to.

    The value returned has type float.

    Precondition: amount_from is a float. Both currency_from
    and currency_to are strings with valid three-letter currency
    codes."""

This function will involve several steps. You will get the JSON string from the web service, break up the string to pull out the numeric value, and then convert that value to a float. As this is the very first assignment, we are going to take you through this process step-by-step.

This assignment might feel like you are working in reverse. You will write the functions to break up the string first, and the functions to interact with the web service last. This is because we want you to develop the following programming habit: always complete and test the helper functions before finishing the functions that use them.

Currency Exchange Table

In order to make it easier to test your program, we have fixed the exchange rates in our web service. That way you can test the answer in a web browser (using a currency query URL) and then compare the results to your Python program, without worrying about rates fluctuating.

The following currencies are supported by our web service:

Note however, that you should not use this table in any of the functions that you write in a1.py. The table above is for testing your functions; not for writing them. There is no reason for you to waste your time hard-coding in all of the currencies listed in this table into your program, since the web service you will contact already knows them all anyway.

Iterative Development (How to Work Through the Assignment)

One of the most important outcomes of this assignment is that you understand the importance of testing. This assignment will follow an iterative development cycle. That means you will write a few functions, then fully test them before you write any more. This process makes it easier to find bugs; you know that any bugs must have been part of the work you did since the last test.

In this section we help you get started with this process. We also provide an overview of the rest of the assignment.

Setting up Python

To do this assignment, Python must be set up properly. If you have not already done this, follow the installation instructions to set it up on your computer. Alternatively, you can just work in the ACCEL lab.

You should also create a folder on your hard drive that is dedicated to this assignment and this assignment only. Every time that you work on a new assignment, we want you to make a new folder, to keep things organized and avoid problems with naming collisions. Make sure that the command shell and Komodo Edit are both open in the current folder before you start.

The Module a1

In your newly created directory, you should create the module a1 (with file name a1.py). This will be the main module for this assignment. Following the style guidelines, the first three lines of this file should be single-line comments with (1) the module name, (2) the name and netid of the authors, and (3) the date the file was last editted. Immediately after this, add the following docstring:

"""Module for currency exchange

This module provides several string parsing functions to
implement a simple currency exchange routine using an
online currency service. The primary function in this
module is exchange()."""

This docstring is the module specification. We recommend that you cut-and-paste the docstring into a1. For now, we want to expose you to specifications, not have you write them on your own.

The Module a1test

Iterative development hinges on proper unit testing, which was covered in lecture and lab. In the same folder as a1.py, create the module a1test (with file name a1test.py). This will be the unit test for the a1 module.

As with a1.py, the first three lines of this file should be single-line comments with (1) the module name, (2) your name and netid, and (3) the date the file was last editted. Immediately after this, add the following Python code:

"""Unit test for module a1

When run as a script, this module invokes several
procedures that test the various functions in the module
a1."""

import cornelltest
from a1 import *
import a1

If you don't have the cornelltest module, you can get it here. As in lab 3 , the module cornelltest provides access to the functions assert_equals and assert_true. The second import statement allows the unit test to access all of the functions in a1, and do it without putting the name in front of each function. [As discussed in lecture, since it is so important to understand where everything is, it is best at this point to explicitly include the module name in calls to things in the module.]

Add four procedure stubs to this assignment: testA, testB, testC, testD. Remember that a procedure stub should have the keyword pass (indented) after the header, but nothing else. We will add our test cases to these procedures later.

Finally at the end of a1test, add the following script code:

if __name__ == '__main__':
    testA()
    testB()
    testC()
    testD()
    print "Module a1 passed all tests"

The script code will call your four test procedures, which are (currently) empty. If everything is working, then the module print out the message

"Module a1 passed all tests"

Try this out.

Instructions for the Remainder of the Assignment

The rest of the assignment is broken into four parts (listed as Parts A, B, C, and D). In each part, do the following:

Write a function header into a1

We will give you the header to write. We will also give you a detailed docstring specification for the function. You should copy-and-paste the specification into the function body, indented.

Add test cases to a1test

Yes, this means you are writing tests before writing the function bodies. We talked about this in lecture.

Unless otherwise instructed, each test case should be a call to an assert function in cornelltest. Furthermore, your tests should be representative. Refer back to the instructions for lab 3 if you do not understand what we mean by this.

Write the function bodies

Make sure that the function satisify the specifications exactly. If the specification says to return something, you need a return statement. Make sure that the value returned is of the correct type.

Run the unit test a1test

If errors are found, fix them and re-test. Keep doing this until no more errors are found.

Writing Function Specifications

The descriptions that we provide in each part below represent the level of completeness and precision we are looking for in your docstring comments. In fact, it is best to copy-and-paste these descriptions to create the first draft of your docstring comments. If you do not cut and paste, please adhere to the conventions we use, such as using a single line, followed by a blank line and a more descriptive paragraph, or by using "Returns: ..." for fruitful-functions. Using a consistent set of good conventions in this class will help us all.

If you want to see if your specifications are written correctly, start an interactive Python shell and type

>>> import a1
>>> help(a1)

Part A: Breaking Up Strings

A large part of this assignment is breaking up a JSON string-value pair. Conceptually, you want to separate the string "v" and the numeric currency amount from the pair. For example, if we are given the string

  '"v": 1.88608072500000'

Then we want to break it up into '"v"' and '1.88608072500000'.

This is the motivation for the after_space function below.

after_space(s)

Returns: Substring of s after the first space

Precondition: s has at least one space in it

Implement this function according to the specification, as described in the Instructions for the Remainder of the Assignment. In other words,

• Write the header and specification in a1.py
• Place test cases in the procedure testA() of a1test.py
• Implement the function in a1.py.
• Test for and correct errors until no errors remain.

To test the functions, you should make use of assert_equals in the module cornelltest to compare the result of each function with the string that you expect to get back. Our unit test has four test cases for each of the two functions the function above. When you think about what test cases you want to include, consider the following:

• Does the specification allow for strings with more than one space?
• Does it allow for strings that start with a space?

Do not forget to add a specification to testA(). Just because it is used for testing does not mean that it should not be properly specified.

The implementation of after_space() should be relatively simple. We were able to implement it in one or two lines.

Part B: Processing a JSON String

All of the valid responses to a currency query contain the keyword string '"v"'. Our goal here is to extract the string-value pair containing these keywords from the response string. For example, from a given response JSON string

  '{"to": "EUR", "rate": 0.75443, "from": "USD", "v": 1.88608072500000}'

  "v": 1.88608072500000

Note that the string-value pair is the substring starting from a double quote '"' of the keyword string, and before the terminating right brace '}'.

While working on each of the functions below, remember to write the test cases in a1test.py before implementing the body. All test cases in this section go in the procedure testB(), which you should remember to specify. You should thoroughly test each function before implementing the next one.

index_keyword_v(s)

Returns: The index of the keyword string '"v"' including the two (double) quote characters

A quote character is one that is inside a string, not one that delimits it. We typically use single quotes (') to delimit a string if want to use a double quote character (") inside of it.

Example: If s is 'A "v" B', this function returns 2
Example: If s is 'A v B "v" C', this function returns 6 because it only picks the keyword v with the quotes around it.

Precondition: s is a string with the keyword string '"v"' inside.

Once you have this function completed, you should move on to the following functions.

get_error(query)

Returns: The boolean value that tells whether a currency query is invalid.

Given a JSON response to a currency query, this returns the boolean value that tells whether a currency query is invalid. If the response is a string of the form

  '{"err": "<error-description>"}'

  '{"err": "failed to parse response from xe.com."}'

Precondition: query is the response to a currency query, contains 'err' string-value pair if and only if query is invalid.

get_v(query)

Returns: The float-type v value of keyword "v" in the response to a currency query.

Given a JSON response to a currency query, this returns the numeric value following the keyword v. For example, if the JSON is

  '{"to": "EUR", "rate": 0.75443, "from": "USD", "v": 1.88608072500000}'

Precondition: query is the response to a valid currency query. 'v' string-value pair always comes last

As always, write your unit tests before implementing the two functions. Look carefully at the specifications. You only need to test valid JSON queries. To get some JSON responses for testing, enter a query URL into the web service and copy the result into a test case.

You should not need a conditional statement to implement these functions; simply find the position of the appropriate keyword and extract the value in quotes immediately after it. Your implementation must make use of the find() or index() string methods, plus the helper function after_space().

Part C: Currency Query

Now it is time to interact with the web service. In this part, you will implement a single function. The test cases for this function should go in procedure testC() in a1test.py. Do not forget to specify testC() properly.

currency_response(amount_from, currency_from, currency_to)

Returns: A JSON string that is a response to a currency query.

A currency query converts amount_from money in currency currency_from to the currency currency_to. A valid response should be a string of the form

  '{"to": "<original-code>", "rate": <conversion-rate>, "from": "<target-code>", "v": <new-amount>}'

  '{"err": "<error-description>"}'

Precondition: amount_from is of type float, while currency_from and currency_to are of type string

While this function sounds complicated, it is actually the simplest function so far and can be implemented in two lines. You need to use the urlopen function from the module urllib2. This function takes a string that represents a URL and returns an instance of the class addinfourl that represents the web page for that url. This object has the following methods:

Using one or both of these methods (you might not need them both) is enough to implement the function above.

Testing

You need to ensure that the function returns exactly the right JSON string for the value given. The best way to test this is to use a web browser to manually get the right JSON answer. such as For example, one test case can be constructed by seeing the result of going to the URL http://cs1110.cs.cornell.edu/2014sp/a1/calculator.php?from=USD&to=EUR&q=2.5 Copy the value from this web page into a test case in testC(). Then check that the function returns the same JSON string. Remember to be thorough with your choice of test cases; one is not enough.

Important: Fetching a web page takes time, especially if too many people are trying to do so at the simultaneously. You should give each call to this function at least 5-10 seconds to complete before restarting any tests.

Part D: Currency Exchange

We are now ready for the final part of the assignment. Implement the following specifications, again using our test-case-before-function-body approach. The test cases should go in procedure testD() in a1test, which you should properly specify. You may wish to use assert_true() instead of assert_equals() in some of your test cases. As with lab 3, there is also a case in which you will want to use assert_floats_equal().

iscurrency(currency)

Returns: True if currency is a valid (3 letter code for a) currency. It returns False otherwise.

Precondition: currency is a string.

In implementing iscurrency(), you should not use the table of currencies. That would make a very large function with a lot of if-statements. You are not allowed if-statements in this lab. Instead, you must use the functions currency_response and get_error() as helper methods .

exchange(amount_from, currency_from, currency_to)

Returns: amount of currency received in the given exchange.

In this exchange, the user is changing amount_from money in currency currency_from to the currency currency_to. The value returned represents the amount in currency currency_to.

The value returned has type float.

Precondition: amount_from is a float. Both currency_from and currency_to are strings with valid three-letter currency codes.

Testing

In the case of iscurrency(), you will find the exchange table useful in determining correct answers for your test cases. While it is not okay to use the table in the body of iscurrency() itself, it is okay to use the table to to decide on some test cases.

You may also use the table to craft some test cases for the function exchange. However, you might find it easier to use a currency query URL to look up the correct answer, and then paste the answer into your test case.

A bigger issue with testing exchange is that problem that we saw in class: real numbers cannot be represented exactly. This creates problems when you try to test equality between floats. To solve this problem, cornelltest provides a function called assert_floats_equal(), which you encountered in lab. You should use this function to test exchange() instead of assert_equals().

Finally, bear in mind that, like currency_response, these functions connect to the web service, and so are not instantaneous. In our solution, with complete test procedures for everything, it can take up to 5 seconds to run the unit test on campus. This will be a bit slower if you are working closer to the deadline.

Finishing the Assignment

Once you have everything working you should go back and make sure that your program meets the class coding conventions, including the following:

1. There are no tabs in the file, only spaces.
2. Functions are each separated by two blank lines.
3. Lines are short enough (~80 characters) that horizontal scrolling is not necessary.
4. The specifications for all of the functions are complete.
5. Function specifications are immediately after the function header and indented.
6. Your name(s) and netid(s) are in the comments at the top of the modules.

In the case of long lines, note that Python allows you to "hit Return" within any expression inside of parentheses. So if you have a long string expression of the form

string1 + string2 + ... + stringk

(string1 + string2 + ... + stringi+
      stringj + ... + stringk)

Turning it In

Upload the files a1.py and a1test.py to CMS by the due date: Tuesday, February 25th at 11:59 pm. Do not submit any files with the extension/suffix .pyc. It will help to set the preferences in your operating system so that extensions always appear.

Check the CMS daily until you get feedback from a grader. Make sure your CMS notifications for CS 1110 are set so that you are sent an email when one of your grades is changed. To find the feedback, click on the Assignment 1 link in CMS. On the page you are brought to, click on the red word "show" in the line "Grading Comments & Requests (show)." You can contact your grader if you have questions about their feedback; you can see their netid in the place you can see their feedback.

Within 24 hours, do RRRRR: Read the feedback, Revise your program accordingly, Resubmit, and Request a Regrade using the CMS. If you do not request a regrade, we have no simple way of knowing that you have resubmitted.

This whole process, starting from first submission on Tuesday, February 25th, continues until you submit a solution that demonstrates complete mastery; in some cases this may require multiple additional resubmits by you. You need to complete this process within one week. You need to have submitted a final, correct version by Tuesday, March 4th, which means you will probably want to have re-submitted at least once before then.

Appendix: Connecting to Rate-Exchange Service

This section is not part of the assignment. It is optional. Furthermore, do not make the changes in this section to the file that you submit for grading. It will be sent back to you to fix.

So far you have worked with a simulated currency exchange service. But with a few changes you can use the real thing. In the web service instructions we told you to use the URL prefix

  http://cs1110.cs.cornell.edu/2014sp/a1/calculator.php?

  http://rate-exchange.appspot.com/currency?

Run the exchange function four or five times. See the value being different from our currency table? That is one of the reasons we did not use the service; it is too hard to test against. In fact, even professional software engineers would do what we did: write a program against an unchanging exchange service before deploying it against the real thing.