1. Getting Started

1.1. Running Python Interpreter

Python comes with an interactive interpreter. When you type python in your shell or command prompt, the python interpreter becomes active with a >>> prompt and waits for your commands.

$ python
Python 3.7.4 (v3.7.4:e09359112e, Jul  8 2019, 14:54:52)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

Now you can type any valid python expression at the prompt. python reads the typed expression, evaluates it and prints the result.

>>> 42
42
>>> 4 + 2
6

Problem 1: Open a new Python interpreter and use it to find the value of 2 + 3.

1.2. Running Python Scripts

Open your text editor, type the following text and save it as hello.py.

print("hello, world!")

And run this program by calling python hello.py. Make sure you change to the directory where you saved the file before doing it.

$ python hello.py
hello, world!

1.3. Datatypes

Python has support for all basic datatypes and also have very powerful compound datatypes.

Python has integers.

>>> 1 + 2
3

Python is pretty good at handling very large numbers as well. For example, let us try to compute 2 raises to the power of 1000.

>>> 2 ** 1000
10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376

That is a pretty big numbers, isn’t it? Can you count how many digits it has?

Python has floating point numbers.

>>> 1.2 + 2.3
3.5

Python has strings.

>>> "hello world"
'hello world'
>>> print("hello world")
hello world

String can be enclosed either in single quotes or double quotes. Both are exactly the same. In Python, strings are very versatile and it very easy to work with them.

>>> 'hello' + 'world'
'helloworld'

>>> "hello" * 3
'hellohellohello'

>>> print("=" * 40)
========================================

The built-in function len is used to find the length of a string.

>>> len('helloworld')
10

Python supports multi-line strings too. They are enclosed in three double quotes or three single quotes.

text = """This is a multi-line string.
Line 2
Line 3
and the text may have "quotes" too.
"""
>>> print(text)
This is a multi-line string.
Line 2
Line 3
and the text may have "quotes" too.

Python supports the usual escape codes. \n indicates new line, \t indicates a tab etc.

>>> print "a\nb\nc"
a
b
c

Python has lists. Lists are one of the most useful data types Python.

>>> x = ["a", "b", "c"]
>>> x
['a', 'b', 'c']
>>> len(x)
3
>>> x[1]
'b'

Python has another datatype called tuple for representing fixed width records. Tuples behave just like lists, but they are immutable.

>>> point = (2, 3)
>>> point
(2, 3)

When writing tuples, the parenthesis can be omitted most of the times.

>>> point = 2, 3
>>> point
(2, 3)

It is also possible to assign a tuple multiple values at once:

>>> yellow = (255, 255, 0)
>>> r, g, b = yellow
>>> print(r, g, b)
255 255 0

Python has a dictionary datatype for representing name-value pairs.

>>> person = {"name": "Alice", "email": "alice@example.com"}
>>> person['name']
'Alice'
>>> person['email']
'alice@example.com'

Python has a set datatype too. A set is an unordered collection of elements.

>>> x = {1, 2, 3, 2, 1}
>>> x
{1, 2, 3}

Python has a boolean type. It has two special values True and False to represent truth and false.

Finally, Python has a special type called None to represent nothing.

>>> x = None
>>> print(x)
None

Now you know most of the common data structures of Python. While they look very simple, mastering them takes a bit of practice. Make sure you go through all the examples and the practice problems in the subsequent sections.

1.4. Variables

You’ve already seen variables in the previous section. Let us look at them closely now.

In Python, variables don’t have a type. They are just placeholders which can hold any type of values.

>>> x = 5
>>> x
5
>>> x = 'hello'
>>> x
'hello'

It is important to notice the difference between variables and strings. Often new programmers get tricked by this. Can you spot any error in the following example?

name = “Alice” print(“name”)

1.5. Functions

Python has many built-in functions. The print is the most commonly used built-in function.

>>> print('hello')
hello
>>> print('hello', 1, 2, 3)
hello 1 2 3

We’ve also see the len function in the previous sections. The len function computes the length of a string, list or other collections.

>>> len("hello")
5
>>> len(['a', 'b', 'c'])
3

One important thing about Python is that it doesn’t allow operations on incompatible data types. For example:

>>> 5 + "2"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'

That is because it is not possible to add a number to a string. We need to either convert 5 into a string or "2"` into a number. The built-in function ``int converts a string into a number and the str function converts any value into a string.

>>> int("5")
5
>>> str(5)
'5'
>>> 5 + int("2")
7
>>> str(5) + "2"
'52'

1.5.1. Example: Counting the number of digits in a number

Let us write a program to compute number of digits in a number. Let us look at some numbers first.

>>> 12345
12345
>>> 2 ** 100
1267650600228229401496703205376
>>> 2 ** 1000
10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376

We can combile the previously mentioned built-in functions to solve this.

>>> len(str(12345))
5
>>> len(str(2 ** 100))
31
>>> len(str(2 * 1000))
302

1.6. Writing Custom Functions

Just like a value can be associated with a name, a piece of logic can also be associated with a name by defining a function.

>>> def square(x):
...    return x * x
...
>>> square(5)
25

The body of the function is indented. Indentation is the Python’s way of grouping statements.

The ... is the secondary prompt, which the Python interpreter uses to denote that it is expecting some more input.

The functions can be used in any expressions.

>>> square(2) + square(3)
13
>>> square(square(3))
81

Existing functions can be used in creating new functions.

>>> def sum_of_squares(x, y):
...    return square(x) + square(y)
...
>>> sum_of_squares(2, 3)
13

Functions are just like other values, they can assigned, passed as arguments to other functions etc.

>>> f = square
>>> f(4)
16

>>> def fxy(f, x, y):
...     return f(x) + f(y)
...
>>> fxy(square, 2, 3)
13

It is important to understand, the scope of the variables used in functions.

Lets look at an example.

x = 0
y = 0
def incr(x):
    y = x + 1
    return y
incr(5)
print x, y

Variables assigned in a function, including the arguments are called the local variables to the function. The variables defined in the top-level are called global variables.

Changing the values of x and y inside the function incr won’t effect the values of global x and y.

But, we can use the values of the global variables.

pi = 3.14
def area(r):
    return pi * r * r

When Python sees use of a variable not defined locally, it tries to find a global variable with that name.

However, you have to explicitly declare a variable as global to modify it.

numcalls = 0
def square(x):
    global numcalls
    numcalls = numcalls + 1
    return x * x

Problem 2: How many multiplications are performed when each of the following lines of code is executed?

print square(5)
print square(2*5)

Problem 3: What will be the output of the following program?

x = 1
def f():
    return x
print x
print f()

Problem 4: What will be the output of the following program?

x = 1
def f():
    x = 2
    return x
print x
print f()
print x

Problem 5: What will be the output of the following program?

x = 1
def f():
        y = x
        x = 2
        return x + y
print x
print f()
print x

Problem 6: What will be the output of the following program?

x = 2
def f(a):
    x = a * a
    return x
y = f(3)
print x, y

Functions can be called with keyword arguments.

>>> def difference(x, y):
...    return x - y
...
>>> difference(5, 2)
3
>>> difference(x=5, y=2)
3
>>> difference(5, y=2)
3
>>> difference(y=2, x=5)
3

And some arguments can have default values.

>>> def increment(x, amount=1):
...    return x + amount
...
>>> increment(10)
11
>>> increment(10, 5)
15
>>> increment(10, amount=2)
12

There is another way of creating functions, using the lambda operator.

>>> cube = lambda x: x ** 3
>>> fxy(cube, 2, 3)
35
>>> fxy(lambda x: x ** 3, 2, 3)
35

Notice that unlike function defination, lambda doesn’t need a return. The body of the lambda is a single expression.

The lambda operator becomes handy when writing small functions to be passed as arguments etc. We’ll see more of it as we get into solving more serious problems.

Python provides some useful built-in functions.

>>> min(2, 3)
2
>>> max(3, 4)
4

The built-in function len computes length of a string.

>>> len("helloworld")
10

The built-in function int converts string to ingeter and built-in function str converts integers and other type of objects to strings.

>>> int("50")
50
>>> str(123)
"123"

Problem 7: Write a function count_digits to find number of digits in the given number.

>>> count_digits(5)
1
>>> count_digits(12345)
5

Methods are special kind of functions that work on an object.

For example, upper is a method available on string objects.

>>> x = "hello"
>>> print x.upper()
HELLO

As already mentioned, methods are also functions. They can be assigned to other variables can be called separately.

>>> f = x.upper
>>> f()
'HELLO'

Problem 8: Write a function istrcmp to compare two strings, ignoring the case.

>>> istrcmp('python', 'Python')
True
>>> istrcmp('LaTeX', 'Latex')
True
>>> istrcmp('a', 'b')
False

1.7. Conditional Expressions

Python provides various operators for comparing values. The result of a comparison is a boolean value, either True or False.

>>> 2 < 3
True
>>> 2 > 3
False

Here is the list of available conditional operators.

  • == equal to

  • != not equal to

  • < less than

  • > greater than

  • <= less than or equal to

  • >= greater than or equal to

It is even possible to combine these operators.

>>> x = 5
>>> 2 < x < 10
True
>>> 2 < 3 < 4 < 5 < 6
True

The conditional operators work even on strings - the ordering being the lexical order.

>>> "python" > "perl"
True
>>> "python" > "java"
True

There are few logical operators to combine boolean values.

  • a and b is True only if both a and b are True.

  • a or b is True if either a or b is True.

  • not a is True only if a is False.

>>> True and True
True
>>> True and False
False
>>> 2 < 3 and 5 < 4
False
>>> 2 < 3 or 5 < 4
True

Problem 9: What will be output of the following program?

print 2 < 3 and 3 > 1
print 2 < 3 or 3 > 1
print 2 < 3 or not 3 > 1
print 2 < 3 and not 3 > 1

Problem 10: What will be output of the following program?

x = 4
y = 5
p = x < y or x < z
print(p)

The if statement is used to execute a piece of code only when a boolean expression is true.

>>> x = 42
>>> if x % 2 == 0: print('even')
even
>>>

In this example, print('even') is executed only when x % 2 == 0 is True.

The code associated with if can be written as a separate indented block of code, which is often the case when there is more than one statement to be executed.

>>> if x % 2 == 0:
...     print('even')
...
even
>>>

The if statement can have optional else clause, which is executed when the boolean expression is False.

>>> x = 3
>>> if x % 2 == 0:
...     print('even')
... else:
...     print('odd')
...
odd
>>>

The if statement can have optional elif clauses when there are more conditions to be checked. The elif keyword is short for else if, and is useful to avoid excessive indentation.

>>> x = 42
>>> if x < 10:
...        print('one digit number')
... elif x < 100:
...     print('two digit number')
... else:
...     print('big number')
...
two digit number
>>>

Problem 11: What happens when the following code is executed? Will it give any error? Explain the reasons.

x = 2
if x == 2:
    print(x)
else:
    print(y)

Problem 12: What happens the following code is executed? Will it give any error? Explain the reasons.

x = 2
if x == 2:
    print(x)
else:
    x +

1.8. Lists

Lists are one of the great datastructures in Python. We are going to learn a little bit about lists now. Basic knowledge of lists is requrired to be able to solve some problems that we want to solve in this chapter.

Here is a list of numbers.

>>> x = [1, 2, 3]

And here is a list of strings.

>>> x = ["hello", "world"]

List can be heterogeneous. Here is a list containings integers, strings and another list.

>>> x = [1, 2, "hello", "world", ["another", "list"]]

The built-in function len works for lists as well.

>>> x = [1, 2, 3]
>>> len(x)
3

The [] operator is used to access individual elements of a list.

>>> x = [1, 2, 3]
>>> x[1]
2
>>> x[1] = 4
>>> x[1]
4

The first element is indexed with 0, second with 1 and so on.

We’ll learn more about lists in the next chapter.

1.9. Modules

Modules are libraries in Python. Python ships with many standard library modules.

A module can be imported using the import statement.

Lets look at time module for example:

>>> import time
>>> time.asctime()
'Tue Sep 11 21:42:06 2012'

The asctime function from the time module returns the current time of the system as a string.

The sys module provides access to the list of arguments passed to the program, among the other things.

The sys.argv variable contains the list of arguments passed to the program. As a convention, the first element of that list is the name of the program.

Lets look at the following program echo.py that prints the first argument passed to it.

import sys
print(sys.argv[1])

Lets try running it.

$ python echo.py hello
hello
$ python echo.py hello world
hello

There are many more interesting modules in the standard library. We’ll learn more about them in the coming chapters.

Problem 13: Write a program add.py that takes 2 numbers as command line arguments and prints its sum.

$ python add.py 3 5
8
$ python add.py 2 9
11