Introduction to Python

Nate Mara

2014-10-15

A Few Words

Python is a very powerful and easy to use programming language that emphasizes programmer time over computer time. It allows for the programmer to use many popular programming paradigms with ease:

• Object-Oriented
• Purely functional
• Imperative

Let's Get Sarted!

We're going to use some examples that I've prepared.

$ git clone https://github.com/natemara/python-tutorial
$ cd python-tutorial
$ python3.4

Hello World!

Java

public class HelloWorld {
        public static void main(String[] args) {
                System.out.println("Hello, World!");
        }
}

Python

>>> print('Hello, World!')

Variables

Python uses the concept of Duck Typing, coming from the Duck Test.

Basically, this means "If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck." Python's variables have no types assigned to them. Instead, Python stores types in the values stored at each variable. We can use the type() function to get the type of any value.

>>> x = 'Hello, World!'
>>> print(type(x))
<class 'str'>

>>> x = 135
>>> print(type(x))
<class 'int'>

>>> x += 1.5
>>> print(type(x))
<class 'float'>

Defining Functions

Python functions are defined with the def keyword, and instead of using curly braces to show the scope of a block, Python uses indentation.

>>> def multiply():
        return 300*50

Many Python users use 4 spaces to denote an indentation level, but a tab works just as well.

Function Types

Just as variables in Python have no defined type, functions and their arguments can also be of any type. It is important to note that in Python, functions are no different from variables, except they have the type <class 'function'>.

>>> def my_great_function(x, y):
        print(x + y)
        return x + y

>>> type(my_great_function)
<class 'function'>

>>> some_value = my_great_function(10, 20)
>>> print(some_value)
30
>>> type(some_value)
<class 'int'>
>>> some_value = my_great_function('Hello ', 'World!')
>>> print(some_value)
Hello World!

>>> type(some_value)
<class 'str'>

Conditionals

Conditional statements are largely the same as in Java or C++, with the exception of not requiring parenthesis around the condition, and having an explicit else if statement called elif. Just like functions, the bodies of conditional statements are required to be indented. Also, instead of || or &&, Python uses the words or, and and.

>>> x = 100
>>> if x >= 10000:
        print('That is a big number!')
elif x > 200 or x > 100:
        print('It is still pretty big.')
else:
        print('Alright, it is a small number.')

Lists

Python default colleciton class is the list. In contrast to the array from C-like languages, a list is dynamic, and can hold values of any type.

>>> values = ['Hello', 130, object]
>>> values.append(50000)
>>> print(values)
['Hello', 130, <class 'object'>, 50000]

For Loops

Since for loops are generally used to iterate through the contents of a collection, Python's for loop has this behavior by default.

>>> for i in values:
        print(i)
Hello
130
<class 'object'>
50000

While Loops

while loops are virtually identical to those in C-like languages, with the exception of the lack of perenthesis around the condition, and the indentation denoting the block.

>>> i = 0
>>> while i < 10:
        print(i)
        i += 2
0
2
4
6
8

Dictionary

A dictionary, or dict is a type of collection where each element is referred to not by a numerical index between 0 and n, but can be referred to by any immutable value, called a key.

>>> values = {
        'green': 1,
        'red': [1, 3, 4],
        3: 'three',
        4: 'four',
        'blue': 5.0
}
>>> print(values['green'])
1
>>> print(values[3])
three
>>> print(values['red'])
[1, 3, 4]

List Comprehensions

One of the most powerful aspects of the Python language is the concept of list comprehensions. A list comprehension is like a formula for building a list.

>>> values = [i for i in range(0, 100) if i % 2 == 0]

Putting it Together

As an exercise, let's write a function that that counts the number of vowels that occur in a string using a list comprehension. Things to know:

• the in keyword
• the sum function

Solution

Iterative

>>> def count_vowels(word):
        total = 0
        for letter in word:
                if letter in 'aeiouAEIOU':
                        total += 1
        return total

Pythonic

>>> def count_vowels(word):
        return sum([1 for i in word if i in 'aeiouAEIOU'])

Classes

Python classes are similar to classes in other languages, except all attributes are public by default. There are some slightly different semantics:

• Instance methods must have self as the first parameter
• Instance variable must be referenced as self.*
• The constructor is called __init__()
• toString() is replaced with __str__()

>>> class Fraction():
        def __init__(self, numerator, denominator):
                self.numerator = numerator
                self.denominator = denominator
        def __str__(self):
                return '{:.2f}/{:.2f}'.format(self.numerator, self.denominator)

Source Code Files

• .py extension
• exactly the same syntax as the interactive mode
• hello.py:

print('Hello World!')