• Python
• Python Basics
• More on Python

❖ Python

Python is a very popular programming language

• easy to learn/use  
• with a wide range of useful libraries

We assume that you know enough Python to manipulate DBs

• the primary goal is Database, not Python programming

If you're not overly familiar with Python ...

• there will be many examples of Python code in this course
• there are many excellent tutorials online
• some of this content was "borrowed" from COMP1531 lectures

❖ Python (cont)

Unlike C, Python is an interpreted language

Such languages are often called "scripting languages"

❖ Python (cont)

Python has an interactive interface (like psql)

$ python3
Python 3.7.3 (default, Jul 25 2020, 13:03:44) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" ...
>> print("Hello, world")
Hello, world
>> quit()
$

Or you can run programs that are stored in files

$ echo 'print("Hello, world")' > hello.py
$ python3 hello.py
Hello, world
$

❖ Python Basics

Like C, Python programs consist of

• expressions,  statements,  control structures,  function definitions, imports, ...

Unlike C, Python uses indentation to indicate code nesting, e.g.

Python                    C

if Condition:             if (Condition ) {
   Statements1               Statements1
else:                     } else {
   Statements2               Statements2
Next Statement            }
                          Next Statement

❖ Python Basics (cont)

Comments are introduced by #, to end of line

Data types and constants:

• booleans, e.g.   True, False
• numbers, e.g.   1, 42, 3.14, -5
• strings, e.g.   "a string", "string2", 'it\'s fun'
• lists, e.g.   [1,4,9,16,25], ['a','b','c']
• tuples, e.g.  (3,5), (1,'a',3.0)
• dictionaries, e.g.   {'a': 5, 'b': 98, 'c': 99}

Assignment is via =,   "the usual" operators are available

❖ Python Basics (cont)

Example operators and expressions

name = "Giraffe"
age = 18
height = 2048.11 # mm
 
print(name + ", " + str(age) + ', ' + str(height))
print(f"{name}, {age}, {height}")
print(type(name))
print(type(age))
 
n = 16 // 3
print(f"3 ** 3 == {3 ** 3}")
print(f"16 / 3 == {16 / 3}")
print(f"16 // 3 == {n}")

❖ Python Basics (cont)

Defining functions

# recursive factorial

def fac(n):
   if n <= 1:
      return 1
   else:
      return n * fac(n-1)

print('5! =',fac(5))

❖ Python Basics (cont)

Defining functions (cont)

# iterative factorial

def faci(n):
   f = 1
   for i in range(1,n):
      f = f * i
   return f

print('6! =',faci(6))

A collection of related functions can be packaged into a module

❖ Python Basics (cont)

C programs can import library definitions, e.g.

#include <stdlib.h>
#include <stdio.h>

Python programs can import external modules  (module = collection of definitions)

import sys
import psycopg2
import sound.effects.echo
from sound.effects import echo

Packages (e.g. sound) are collections of sub-packages and modules

❖ Python Basics (cont)

Example: echo in Python

import sys
                    # extracts a slice from argv
for arg in sys.argv[1:] :
   print(arg, end=" ")   # don't put '\n' after print
print("")

which is is used as (if placed in file echo.py)

$ python3 echo.py arg1  "arg 2"  arg3
arg1 arg 2 arg3 
$ 

❖ Python Basics (cont)

Example: sequence generator ... 1 2 3 4 5 ...

#!/usr/bin/python3
import sys
if len(sys.argv) < 3:
   print("Usage: seqq lo hi")
   exit(1)
hi = int(sys.argv[2])
i = lo = int(sys.argv[1])
while i <= hi:
   print(i)
   i += 1   # no ++ operator

which can be used as which is is used as (if placed in executable file seqq)

$ ./seqq 2 4
2
3
4
$

❖ More on Python

Lots of info available on python.org

• including   docs.python.org/3/tutorial/introduction.html

And many others, e.g. www.learnpython.org

Or ask for "free python3 tutorials" on Google

Python has hundreds of modules/libraries on all kinds of topics

We focus on the psycopg2 database connectivity module