Introduction to Python Dr Jos M Reyes lamo

Introduction to Python Dr. José M. Reyes Álamo

Three Rules of Programming • Rule 1: Think before you program • Rule 2: A program is a human-readable set of instructions that solve a problem and can be executed on a computer • Rule 3: The best way to improve your programming and problem solving skills is to practice. 2

Computer Programs • A program is a sequence of instructions. • To run a program is to: – Create the sequence of instructions according to your design and the language rules – Turn that program into the binary code that the computer understands – Give the binary code to the Operating System (OS), so it can pass it to the processor for execution – OS and processor together, execute the program 3

Interpreted • Python is an interpreted language • Interpreted means that Python looks at each instruction, one at a time, and translate it into machine language. – That allows you to enter instructions one-at-a-time. • You can also create a file with several instructions called a script, load it, and execute all the instruction one after the other. • To rerun an script, reload it. 4

Interactive mode vs scripting mode

Two ways to work: Interactive and scripting • One of the benefits of working with an interpreted language is that you can test bits of code in interactive mode before you put them in a script. • But there are differences between interactive mode and script mode that can be confusing. 6

Interactive mode • For example, if you are using Python as a calculator, you might type >>> miles = 26. 2 >>> miles * 1. 61 42. 182 7

Scripting mode • But if you type the same code into a script and run it, you will get no output. • In script mode an expression, has no visible effect. • Python actually evaluates the expression, but it doesn’t display the value unless you tell it to do so: miles = 26. 2 print miles * 1. 61 8

Calculating area and circunference

Code Example The Practice of Computing Using Python, Punch, Enbody, © 2011 Pearson Aaddison -Wesley. All rights reserved 10

Import of Math • One thing we did was to import the math module with import math • This imported python math statements • We precede all operations of math with math. OPERATION – e. g. math. pi, math. pow(x, y) 11

Getting Input The function: input(“Give me a value”) • Prints “Give me a value” on the python screen and waits until the user types anything and presses Enter • After pressing enter, the result is stored as a string 12

Assignment Statements • The ‘=‘ is the assignment statement • The value to the right is associated with the variable name on the left • It does not stand for equality! 13

Convert from string to integer • To do math, Python requires converting the sequence of characters into an integer 14

Printing Output my. Var = 12 print (‘My var has a value of: ’, my. Var) • print takes a list of elements to print, separated by commas – If the element is a string, prints it as is – If the element is a variable, prints the value associated with the variable – After printing, moves on to a new line of output 15

Save as a script • When you save a file with a. py extension, it becomes a python script • You can run the script from the IDLE to see the results of the operation • A script is just a file with several python commands • Python provides many scripts or modules for common tasks such as math, databases, networking, etc. 16

Errors!!! • If Python interpreter does not understand your code you will get errors • Check the syntax and fix it • You can them import the program again until there are no more errors 17

Common Error • When using IDLE, if you save a file without a. py extension, it will not colorize and format the file. • Resave with the. py extension 18

Statements • Statements are commands. – They perform some action n = n + n ** 3 n = 3 print ‘Hello!’ 19

Expressions • Expressions perform some operation and have a value – Typically do not modify values in the interpreter 3 n + 3 n ** 3 20

Comments • Code is what actually executes • Comments do not execute but they make the programs easier to understand, improving readability. n + 3 n = n ** 3 #this an expression #this is an statement print ‘Hello!’ #this is another statement 21

Literals • Literal is a programming notation for a fixed value. • Integer numbers are literals because they have fixed values – e. g. 123, 999, 1000. 22

Operators • Integer – Addition and subtraction: +, - – Multiplication: * – Division (quotient): / – Remainder: % • Floating point – add, subtract, multiply, divide: +, -, *, / 23

Variables • A variable is a location in memory where we store data in our program. • We assign names to variables to make our program more readable. 24

Variables • Python maintains a list of pairs for every variable: – variable’s name – variable’s value • A variable is created when a value is assigned the first time. It associates a name and a value • Subsequent assignments update the associated value. • We say the variable name references the value • A variable type depends on the value assigned to it. X=7 Name Value X 7 25

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Python Naming Conventions • Must begin with a letter or an _ (underscore) – Ab 123 and _my. Data are OK, but 123 ABC is not. • May contain letters, digits, and underscores – this_is_an_identifier_123 • May be of any length • Upper and lower case letters are different – Length. Of. Rope is not the same as lengthofrope • Variables starting with _ have special meaning. 27

When = Doesn’t Mean Equal • It might look confusing at first to see the following expression: – my. Int = my. Int + 7 • Here the = doesn’t mean equal, but assignment. 28

= sign means assignment • In many computer languages the = means assignment. – my. Int = my. Int + 7 – lhs = rhs • What assignment means is: – Evaluate the expression on the right-hand-side of the equal sign – Take the resulting value and store it in the variable indicated on the left-hand-side • Only variables can go into the left-hand side of an assignment statement 29

More on Assignment • Example: x = 3 * 5 + 2 – evaluate expression (3*5 + 2) = 17 – change the value of x to 17 • Example (if y has value 2): y = y + 3 – evaluate the expression (y+3) = 2+ 3 = 5 – change the value of y to 5 30

Variables and Types • Python does not require you to pre-define the data type of a variable • The data type of a variable can change • Nonetheless, knowing the data type can be important for using the correct operation on a variable. Thus proper naming is important! 31

What is a Type? • A Python data type essentially defines two things: – The internal structure of the type (what it contains) – The operations you can perform on it. • You can capitalize letter, and add numbers – You cannot capitalize numbers or add letter. 32

Python Data Types • Integer: 5 • Float: 1. 2 • Boolean: True, False • String: “anything” or ‘something’ • List: [, ]: [‘a’, 1, 1. 3] • Others 33

Fundamental Types • Integers – 1, -27 • Floating Point – 3. 14, 10. 0, . 001, 3. 14 e-10, 0 e 0 • Boolean (True or False values) – True, False (notice the use of caps) 34

Converting Types • A character ‘ 1’ is NOT the same as the integer 1 • You need to convert the value returned by the input command (characters) into an integer • int(“ 123”) yields the integer 123 35

Type Conversion • int(some. Var) converts to an integer • float(some. Var) converts to a float • str(some. Var) converts to a string • should check out what works: – int(2. 1) 2, int(‘ 2’) 2, but int(‘ 2. 1’) fails – float(2) 2. 0, float(‘ 2. 0’) 2. 0, float(‘ 2’) 2. 0, float(2. 0) 2. 0 – str(2) ‘ 2’, str(2. 0) ‘ 2. 0’, str(‘a’) ‘a’ 36

Types and Division Python does binary operations on two values of the same type, yielding a value of that type: • 2/3, integer types, yield integer (0). – 2%3 is the remainder, an integer (2) • 2. 0/3. 0, float types, yield float (0. 66666) 37

Mixed Types • 4/3 results is 1 under integer division • 4. 0/3. 0 results in 1. 3333333 under float division • What is the result of 4/3. 0? – Python will automatically convert to the most detailed type. Thus 4 4. 0, and the result is 1. 3333333 38