Homework 1: Intro to Python and Github

This assignment is due at 11:59pm on Friday, January 24.
This class requires a POSIX environment for the programming assignments and we suggest you get things set up early on.

This assignment has the following purposes:

  1. To get a Python 3.5+ environment installed on your system
  2. To familiarize you with the OK submission and grading environment
  3. To do some basic tasks in python to make sure you’re familiar with the language’s basic syntax and features.

Introduction to Python

For this class Python 3.5 or above is required. Please make sure that you are using Python 3.5+ for all assignments. Python 3 is similar to Python 2, but the two are incompatible in many ways. Programs that work on Python 2 may not work on Python 3. You will be responsible for making sure your assignments work with Python 3.6.

If you’re unsure what version of python you are using, you can always check by typing python3 --version on the command line.

Python is a popular, dynamic, memory managed language. It has many features that make it easier to work with than other languages. If you’re already familiar with python and have it installed, feel free to skip over this section. For those new to the language, this section provides an overview of some of the unique features of the language.

Installing Python

You should make sure you have Python 3.5+ installed on your computer. You can get the latest version of python by going to the Python website and installing the appropriate download. If you have any questions or problems in the installation process, please contact the TA.

Python Basics

Syntax

If you’re used to languages like C and Java, python might look a little alien. This is because python does not use curly braces ({ or }) to denote the end of blocks, or semi-colons to denote the end of lines. Instead, Python relies on white space.

// In C you end up repeating yourself a lot, indenting things and wrapping
// blocks in curly braces.
void a_function(int * firstArg, int * secondArg) {
    // You also repeat yourself, but using both ";" and new lines
    // to mark the end of a line of code.
    int firstInt = 1;
    int secondInt = 2;
}

# In Python, you type less, and have less redundancy.  You
# may find that this reduces the number of errors you make, and makes
# your programs easier to maintain.
def a_function(first_arg, second_arg):
    first_int = 1
    second_int = 2

Dynamic Types

Languages like Java and C require you to declare types in function signatures and variable declarations. If, for example, you’re writing a program C, you and you want to declare a variable to be an integer, and another variable to be a character, you need to do something like the following:

#include <stdio.h>

// Defining some variables in C
int myInt = 3;
char someChar = 'a';

// Defining a function in C
int some_function(int anInt, char aChar) {
    printf("Here is the integer I was given: %d\n", anInt);
    printf("And here is the character I was given: %c\n", aChar);
    return 0;
}

// C programs always start with the main function
int main(int argc, char * argv[]) {
    // Calling a function in C
    some_function(myInt, someChar);
}

In most ways, python is much less strict about types. You don’t specify a type when you declare a variable, and variables can take on values of any type, even changing over the lifetime of your program. The below, for example, is valid python code:

# Defining some variables in python.  Note that we don't define any
# types, python does that automatically.
my_int = 3
some_char = "a"

# Defining a function in python.  Note that you don't declare types
# for the arguments in python, the language handles that for you.
# We also don't have to declare a return type, thats handled automatically
# too.
def some_function(an_int, a_char):
    print("Here is the integer I was given:", an_int)
    print("And here is the character I was given:", a_char)

some_function(my_int, some_char)

Native Data Structures

Python provides many conveniences for defining and working with data structures compared to languages like Java and C. Python doesn’t allow you to do anything you can’t do in Java and C, Python makes it much easier, and much less error prone, to interact with structures like arrays (which python calls lists), hash tables (which python calls dicts), and sets (which python calls… sets).

# In python, you can define an array (or, in python terms, a list) of
# variables in line, and then loop over them easily.  The below code
# creates a list / array of three strings, and then prints them all out.
list_of_strings = ["first", "second", "third"]

# Python provides this short hand syntax for iterating over each element
# in a list.  In C, you might use a for loop and index into each element
# in the list.
#
# The below list will print out the following text:
#
#   first
#   second
#   third
for a_string in list_of_strings:
    print(a_string)

# We can do the same thing with a hash table, or a mapping of values
# to values.  The below code maps the names of numbers (as strings) to
# their actual value (as integers).
number_mapping = {
    "one": 1,
    "two": 2,
    "three": 3,
}

# Now we loop over each item in the hash table (or dict) we created,
# which will produce the following lines (though the order is
# unpredictable).
#
#   one can also be written as 1
#   two can also be written as 2
#   three can also be written as 3
for string_version, int_version for number_mapping.items():
    print(string_version, "can also be written as", int_version)

Large Standard Library

Python comes with a large standard library, and includes “out of the box” a large amount of functionality to help you complete common tasks. This is true compared to Java, and especially compared to C. This reduces the amount of third party code you need in Python to complete common tasks.

A full list of all the functionality included in the python standard library can be found online.

Other Python Features

The above just scratches the surface of what makes python an interesting and useful language. Python has powerful object oriented tools like Java, closures and anonymous functions like Lisp and JavaScript, and a full module system for structuring your code and taking advantage of code written by others.

You wont be using most of these features in this class, but they’re in the language and it will benefit you to learn more about them as you become more familiar with the language.

Writing Better Python

There are great tools to help you become a better python programmer, and to help you write better, cleaner, less-bug-laden code. Its highly recommended that you use these tools, and they can help you catch, fix and prevent errors, and write code that will be easier to understand if you ever need to revisit them (plus, if you ever use any of these assignments as code samples in a job application, its a good thing to show that you’re familiar with and already following python best practices.

One tool is pep8, a command line tool that checks that your code is following python formatting and style best practices. Making that your code is well formatted will make it easier to understand, revisit, revise, and help you avoid subtle errors. You should check that your code matches the pep8 style wherever possible.

pylint is another code quality tool. It checks that your code is well documented, well formatted, and avoids practices that can make your code confusing and fragile. You should also consider using pylint to check all the python code you submit in this course.

For this assignment, using pep8 and pylint is not required, just highly recommended. However, it may be required on future assignments. Please familarize yourself with these tools as soon as possible.

Using The Git Environment

For all assignments in this class, you will be using Github Classroom to submit and grade your work. If you have any trouble, please contact the TA.

The specific assignment in github classroom is here. In these assignments, you will be cloning a repository that already contains starter code.

Starter Files

  • hw1.py: The source file you where you will be writing your code. The OK client will only backup/submit this file.
  • tests: A directory containing the test cases for the assignment.
  • data: A directory containing file(s) which you will be using for this assignment.

Submitting Your Work

To submit your work, use git to commit (git commit -a) and push (git push) your changes. For this assignment, there is no limit on the number of submissions you can make. We recommend submitting your work after performing each individual task. It is important to note that automatic backups created do not count as submissions.

Assignment Tasks

Now that you have an understanding of how to use OK for testing and submitting your work, let’s get started with the actual tasks that you will be writing for this assignment. You will only be making changes to hw0.py as this is the only submitted to the server for grading.

  1. Modify the function get_version in hw0.py to return the version of python in the form of 3.x.x . Make sure your return string is a single line and you remove any other details from your response.

  2. Modify the functionalternative_sum in hw0.py so that it takes in 2 integer arguments, start, n and returns a list generated by the following sequence:

    Using start as an initial number in the list, the function increments the list by adding a new element at the end of the list by summing the last and the third last element. If the third last element is the list does not exist, the new element is generated by summing the last element to itself. The function returns the generated list once the length of the list reaches n. In addition, your function needs to checks for the following conditions:

    • start can only be a non zero positive number. If it is negative, the function should return an empty list.
    • n should be a number between 5 and 20, inclusive. If n is out of range, the function should return an empty list.

    For example if I call alternative_sum(15, 5) the function returns [15, 30, 60, 75, 105]. The list is generated as follows:

     [15] # The new element is will generated by summing the last element to itself.
 [15, 30] 
 [15, 30, 60] # New element will now be generated by summing the last and the third last elements.
 [15, 30, 60, 75]
 [15, 30, 60, 75, 105] # final result list to be returned.

  3. The third task of the assignment deals with reading from a file. The data directory contains a file named scores.csv. Your goal in this task is to modify the function order_scores in hw0.py so that it returns the names in the form of a list which is sorted by their respective scores in ascending order. For example, Shawn has the lowest score so will be the first element of the returned list.

Testing Specifics

python3 ok will execute all the tests every time you run it. OK also provides the functionality of only executing specific test cases. For example, to run the cases from the file tests/01.py, run the following command: python3 ok -q 01. Here are a few additional pointers

  • To see results of the test cases in more detail you can also the -v flag.
  • The --score will display how much you scored on each test.
  • To run your tests locally and disable code backing up on the server, you run provide the --local flag.

Hints

For number 1, do not hardcode the version of python into your code (i.e. do not write return(“3.6”)). You must use the python system for return the current version to receive credit. Refer to the sys module, which will be helpful here. Also, depending on your environment, python might print your python version in more than one line. You can use the relevant string methods to remove extra information and achieve the required formatting of the response.

For number 3, there are a few ways you can read the file, specifically, the csv module will be helpful in this case. To make things easier for you, the scores in the file are unique.

Submission and Logistics

  • Remember, once you are done with the assignment submit your work by running git commit -a and git push
  • This assignment is due at 11:59pm on Friday, January 24.
  • No late submissions will be accepted.
  • If you have any questions please use the Piazza discussion forum.