Python first steps

Mithat Konar
based on Dierbach's Introduction to Computer Science using Python

• Syntax is easy to read.
• Interpreted and can be executed interactively.
• Huge community.
  • Lots of support.
  • Lots of open source modules.

• IDLE is Python’s ‘standard’ IDE.
• Good for starting to learn Python.
• Many other IDEs.

• Often called REPL (read-evaluate-print-loop) mode.
• >>> is the shell prompt

>>> 4 + 6
10
>>> 3 * 7
21

• +, -, *, /
• ** is exponentiation
• Note the use of parenthesis in multiplication.

>>> 4 + 6
10
>>> 3 * 7
21
>>> 5 / 2
2.5
>>> 2 * (7 + 3)
20
>>> 2 ** 3
8

• Python’s core is limited.
• Add functionality by importing modules.
• Lots of modules in the standard library.

>>> import math
>>> 4 * math.pi
12.566370614359172
>>> math.factorial(4)
24

• variable: a name associated with a value.

>>> r = 5
>>> 2 * r
10

• Variables can store character string values as well.

>>> city = 'Jakarata'

• Use print() to generate output.

>>> print('Hello, there.')
Hello there.
>>> n = 5
>>> print(n)
5

• Use input() to get input.

>>> name = input('What do I call you? ')
What do I call you? Brian
>>> print('Hi,', name)
Hi, Brian

• Create a new program file with File > New File or Ctrl+N (Linux and Windows).
• Write code:

name = input('What do I call you? ')
print('Hi,', name)
print('Eat more vegetables.')

• Save the file: File > Save (Ctrl+S) or File > Save As … (Ctrl+Shift+S)
  • File extension .py will be added automatically.
• Run: Run > Run Module (F5)

• Comments let you write notes to yourself or the reader of your code.
• Anything after a # will be ignored by the interpreter.

# This is my first Python program.
name = input('What do I call you? ')  # get user's name
print('Hi,', name)  # print name entered by user
print('Eat more vegetables.')

• The + operator can be used to concatenate (put together) two string values.

>>> name = 'Carol ' + 'Cleveland'
>>> print(name)
Carol Cleveland

• Convert a number to a string with str().

>>> x = 99
>>> message = 'Number of red balloons: ' + str(x)
>>> print(message)
Number of red balloons: 99

• Convert a string to an integer with int().

>>> x = '99'
>>> x + 1 # error because x is not a number but 1 is.
>>> int(x) + 1
100

• input() in Python 3 will always return a string.
• If you expect a number, convert it:

num_balloons = input('How many balloons? ')
num_balloons_converted = int(num_balloons) + 1

• Combining getting input and converting:

num_balloons = int(input('How many balloons? '))

• You can print more than one thing with print().
• Separate each thing with a comma.

>>> num = 99
>>> print("There are", num, "balloons.")
There are 99 balloons

• Each “thing” will be converted into a string.

1. Define the problem.
2. Design a solution.
3. Code the solution.
4. Test the solution.
5. Document the solution.

I need an exponent calculator.

• Solution: Write a program that allows the user to enter any integer base and integer exponent, and displays the value of the base raised to that exponent.

• Observations:
  • The user will enter the values from the keyboard.
  • It would be good if the user got feedback when the result is printed.
  • I will use Python because it will be easy to implement the solution with it.
• Proposed interaction:

What base? 10
What power of 10? 4 
10 to the power of 4 is 10000

1. Get the base from the user.
2. Get the exponent from the user.
3. Calculate base to the exponent power.
4. Print “base to the power of exponent is result.”

# Exponent Calculator
# Calculate a base to an exponent's power.
# Mithat Konar
 
base = input('Enter the base: ')
exponent = input('Enter the exponent: ')
result = base ** exponent
print(base + ' to the power of ' + exponent + ' is ' + result)

>>> Enter the base: 10
>>> Enter the exponent: 4
Traceback (most recent call last):
  File "/home/mithat/exponent_calculator.py", line 7, in <module>
    result = base ** exponent
TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'str'

Oops. Back to step 3.

# Exponent Calculator
# Calculate a base to an exponent's power.
# Mithat Konar
 
base = int(input('Enter the base: '))
exponent = int(input('Enter the exponent: '))
result = base ** exponent
print(base + ' to the power of ' + exponent + ' is ' + result)

>>> Enter the base: 10
>>> Enter the exponent: 4
Traceback (most recent call last):
  File "/home/mithat/lesson-dev.py", line 8, in <module>
    print(base + ' to the ' + exponent + 'power is ' + result)
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Oops (again).

# Exponent Calculator
# Calculate a base to an exponent's power.
# Mithat Konar
 
base = int(input('Enter the base: '))
exponent = int(input('Enter the exponent: '))
result = base ** exponent
print(str(base) + ' to the power of ' + str(exponent) + ' is ' + str(result))

>>> Enter the base: 10
>>> Enter the exponent: 4
>>> 10 to the power of 4 is 10000

Success. But we can do better…

# Exponent Calculator
# Exponent Calculator
# Calculate a base to an exponent's power.
# Mithat Konar
 
base = int(input('Enter the base: '))
exponent = int(input('Enter the exponent: '))
result = base ** exponent
print(base, 'to the power of', exponent, 'is', result)

>>> Enter the base: 10
>>> Enter the exponent: 4
>>> 10 to the power of 4 is 10000

The code is easier to read and it still works.

• In this case, we began documenting the program right from the start.
  • Writing out details of the problem.
  • Writing out the design of the solution.
  • Using comments in the source code.
• Do this.
• Every. Single. Time.