2. Compilers and Interpreters
Programs written in high-level languages must be translated into machine language to be executed
Compiler: translates high-level language program into separate machine language program
Machine language program can be executed at any time

3. Compilers and Interpreters (cont’d.)
Interpreter: translates and executes instructions in high-level language program
Used by Python language
Interprets one instruction at a time
No separate machine language program
Source code: statements written by programmer
Syntax error: prevents code from being translated

4. Compilers and Interpreters (cont’d.)
Executing a high-level program with an interpreter

5. Compiling and interpreting
Many languages require you to compile (translate) your program into a form that the machine understands.
Python is instead directly interpreted into machine instructions.
compile
execute
output
source code
Hello.java
byte code
Hello.class
interpret
output
source code
Hello.py

6. Using Python Python must be installed and configured prior to use
One of the items installed is the Python interpreter
Python interpreter can be used in two modes:
Interactive mode: enter statements on keyboard
Script mode: save statements in Python script

7. Interactive Mode
When you start Python in interactive mode, you will see a prompt
Indicates the interpreter is waiting for a Python statement to be typed
Prompt reappears after previous statement is executed
Error message displayed If you incorrectly type a statement
Good way to learn new parts of Python

8. Writing Python Programs and Running Them in Script Mode
Statements entered in interactive mode are not saved as a program
To have a program use script mode
Save a set of Python statements in a file
The filename should have the .py extension
To run the file, or script, type
python filename
at the operating system command line

9. Python Scripts
When you call a python program from the command line the interpreter evaluates each expression in the file
Familiar mechanisms are used to provide command line arguments and/or redirect input and output
Python also has mechanisms to allow a python program to act both as a script and as a module to be imported and used by another python program

10. The IDLE Programming Environment
IDLE (Integrated Development Learning Environment): single program that provides tools to write, execute and test a program
Automatically installed when Python language is installed
Runs in interactive mode
Has built-in text editor with features designed to help write Python programs

11. IDLE Development Environment
IDLE is an Integrated DeveLopment Environ-ment for Python, typically used on Windows
Multi-window text editor with syntax highlighting, auto-completion, smart indent and other.
Python shell with syntax highlighting.
Integrated debugger with stepping, persis- tent breakpoints, and call stack visi- bility

12. Programming basics
code or source code: The sequence of instructions in a program.
syntax: The set of legal structures and commands that can be used in a particular programming language.
output: The messages printed to the user by a program.
console: The text box onto which output is printed.
Some source code editors pop up the console as an external window, and others contain their own console window.

13. The Basics

14. A Code Sample (in IDLE) x = 34 - 23 # A comment.
y = “Hello” # Another one.
z = 3.45
if z == 3.45 or y == “Hello”:
x = x + 1
y = y + “ World” # String concat.
print (x)
print (y)

15. Enough to Understand the Code
Indentation matters to code meaning
Block structure indicated by indentation
First assignment to a variable creates it
Variable types don’t need to be declared.
Python figures out the variable types on its own.
Assignment is = and comparison is ==
For numbers + - * / % are as expected
Special use of + for string concatenation and % for string formatting (as in C’s printf)
Logical operators are words (and, or, not) not symbols
The basic printing command is print

16. Basic Datatypes Integers (default for numbers) Floats Strings
z = 5 // 2 # Answer 2, integer division
Floats
x = 3.456
Strings
Can use “” or ‘’ to specify with “abc” == ‘abc’
Unmatched can occur within the string: “matt’s”
Use triple double-quotes for multi-line strings or strings than contain both ‘ and “ inside of them: “““a‘b“c”””

17. Whitespace
Whitespace is meaningful in Python: especially indentation and placement of newlines
Use a newline to end a line of code
Use \ when must go to next line prematurely
No braces {} to mark blocks of code, use consistent indentation instead
First line with less indentation is outside of the block
First line with more indentation starts a nested block
Colons start of a new block in many constructs, e.g. function definitions, then clauses

18. Comments Start comments with #, rest of line is ignored
Can include a “documentation string” as the first line of a new function or class you define
Development environments, debugger, and other tools use it: it’s good style to include one
def fact(n):
“““fact(n) assumes n is a positive integer and returns facorial of n.””” assert(n>0)
if n==1 return 1 else n*fact(n-1)

19. Assignment
Binding a variable in Python means setting a name to hold a reference to some object
Assignment creates references, not copies
Names in Python do not have an intrinsic type, objects have types
Python determines the type of the reference automatically based on what data is assigned to it
You create a name the first time it appears on the left side of an assignment expression: x = 3
A reference is deleted via garbage collection after any names bound to it have passed out of scope
Python uses reference semantics (more later)

20. Assignment You can assign to multiple names at the same time
>>> x, y = 2, 3
>>> x 2
>>> y 3
This makes it easy to swap values
>>> x, y = y, x
Assignments can be chained
>>> a = b = x = 2

21. Naming Rules
Names are case sensitive and cannot start with a number. They can contain letters, numbers, and underscores.
bob Bob _bob _2_bob_ bob_2 BoB
There are some reserved words:
and, assert, break, class, continue, def, del, elif, else, except, exec, finally, for, from, global, if, import, in, is, lambda, not, or, pass, print, raise, return, try, while

22. Naming conventions
The Python community has these recommend-ed naming conventions
joined_lower for functions, methods and, attributes
joined_lower or ALL_CAPS for constants
StudlyCaps for classes
camelCase only to conform to pre-existing conventions
Attributes: interface, _internal, __private

23. Accessing Non-Existent Name
Accessing a name before it’s been properly created (by placing it on the left side of an assignment), raises an error
>>> y
Traceback (most recent call last):
File "<pyshell#16>", line 1, in -toplevel- y
NameError: name ‘y' is not defined
>>> y = 3 3

24. Expressions
expression: A data value or set of operations to compute a value.
Examples: * 3 42
Arithmetic operators we will use:
+ - * / addition, subtraction/negation, multiplication, division
% modulus, a.k.a. remainder
** exponentiation
precedence: Order in which operations are computed.
* / % ** have a higher precedence than * 4 is 13
Parentheses can be used to force a certain order of evaluation. (1 + 3) * 4 is 16

25. Boolean Operations — and, or, not
Comparisons
Operation
x or y
x and y
not x
Operation
Meaning
<
strictly less than
<=
less than or equal
>
strictly greater than
>=
greater than or equal ==
equal !=
not equal is
object identity
is not
negated object identity

26. Numeric Types — int, float
Operation
Result
x + y
sum of x and y
x - y
difference of x and y
x * y
product of x and y
x / y
quotient of x and y
x // y
floored quotient of x and y
x % y
remainder of x / y -x
x negated +x
x unchanged
abs(x)
absolute value or magnitude of x
int(x)
x converted to integer
float(x)
x converted to floating point
divmod(x, y)
the pair (x // y, x % y)
pow(x, y)
x to the power y
x ** y

27. Math commands Python has useful commands for performing calculations.
To use many of these commands, you must write the following at the top of your Python program:
from math import *
Command name
Description
abs(value)
absolute value
ceil(value)
rounds up
cos(value)
cosine, in radians
floor(value)
rounds down
log(value)
logarithm, base e
log10(value)
logarithm, base 10
max(value1, value2)
larger of two values
min(value1, value2)
smaller of two values
round(value)
nearest whole number
sin(value)
sine, in radians
sqrt(value)
square root
Constant
Description e pi