Download Announcements Python Why Python? What to do today

Announcements
Python
—  Let’s learn programming with a general purpose language (Python).
—  Try to get help from me and tutors
—  After we study Python, we will study JavaScript to continue with
client-side web programming that we started with HTML/CSS.
—  What to do today
—  Start Python
—  After that we will continue with server-side web programming with
Python/Flask.
—  Reading assignment for this slide set: Chapters 1 and 2 of Downey
—  Break around 10:15am
—  Python is much cleaner language to use to learn the concepts of
programming than JavaScript. Also JavaScript is a special purpose
language for web development.
—  We will take a systematic approach in learning a programming
language and the fundamental concepts involved in programming
with Python.
1
2
Why Python?
What to do today
—  Simple, simpler than Java (which is simpler than C++)
"In 2003 I started teaching at Olin College and I got to teach Python for the first time.
The contrast with Java was striking. Students struggled less, learned more, worked on
more interesting projects, and generally had a lot more fun." -- Allen Downey
—  The first Python program!
—  Highly relevant to non-CS majors
—  Python programming environment
—  Python is a general-purpose programming language
—  Python packages such as NumPy, SciPy are heavily used by scientists
—  Variables
—  Other packages are available for various domains
—  Is a modern language with good support
—  Expressions and statements
—  Popular for web applications (e.g., Facebook apps)
—  Fun to program in
—  Strings
—  Widely used: becoming one of the most popular languages
http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html
3
4
1
Getting started with Python
First cup of Python
—  Install Canopy, an IDE (Integrated Development Environment)
using the instructions on the Course Schedule page
—  See simple.py
—  Interactive mode
—  Script mode
—  See ch2.py
—  Can also run from the “command line”
—  Mac: Terminal
—  Windows: Command prompt
—  Start with the python or ipython command
—  Both interactive session and script mode are possible with either
—  We will try all these different ways in class together
—  Python files has the .py extension, e.g., simple.py
—  We will use Python 2.7 - Textbook uses 2.7
5
The basics
6
Representing values
—  Values
—  23
—  12.321
—  “Hello world!”
—  Types
—  integer
—  float (real number)
—  string (of characters)
—  boolean (True and False)
—  Everything on a computer reduces to numbers
—  Letters represented by numbers (ASCII codes, see next slide)
—  Pixel colors are three numbers (red, green, blue)
—  So, how can Python tell all these numbers apart? By type!
—  Type
—  A set of values and the operations on them
—  Examples of operators: +, -, /, *, **, %, etc.
—  The meaning of these operators depends on the type of their operands
—  Expressions (try these on Python interpreter)
—  23 * (12 + 56)
—  3.4/2.3
—  ‘Hello,’ + ‘ world!’
7
8
2
Expression
vs.
Statement
—  Represents something
—  Python evaluates it
—  End result is a value
—  Does something
—  Python executes it
—  Need not result in a value
—  Examples
—  23
—  2.54
—  (4 + 12 * 4) * 2.1
—  Examples
—  print “Hello”
—  x = 3 + 4
9
10
Type: int
Type: float
—  Type int (integer)
—  Values: . . ., -3, -2, -1, 0, 1, 2, 3, . . .
—  Operations: +, -, *, / ,**, %, unary –
—  Type float (floating point number):
—  Values: (approximations of) real numbers
— 
— 
—  Operations: +, -, *, **, unary –
—  Principle: operations on int values must yield an int
—  Example:
— 
— 
— 
In Python a number with a “.” is a float literal, e.g., 3.0
Without a decimal a number is an int literal, e.g., 3
— 
— 
1/2 rounds result down to 0 (it computes the quotient part of division)
Companion operation: % (it computes the remainder part of division)
8 % 5 evaluates to 3, remainder when dividing 8 by 5
The meaning for floats differs from that of ints
Example: 1.0/2.0 evaluates to 0.5 (cf. 1/2 evaluates to 0)
—  Exponent notation is useful for large (or small) values
—  -22.51e6 is -22.51 * 106 or -2251000
—  22.51e-6 is 22.51 * 10-6 or 0.00002251
—  Operator / is not an int operation in Python 3 (use // instead)
11
12
3
Floats have finite precision
Type: str
—  Python stores floats as binary fractions
—  Integer mantissa times a power of 2
—  Example: 1.25 is 5 * 2-2 (5 is mantissa and -2 is power here)
—  Type str for strings
—  Values: any sequence of characters
—  Operation(s): + (concatenation)
—  Impossible to write most real numbers this way exactly
—  Similar to problem of writing 1.0/3.0 with decimals
—  Python chooses the closest binary fraction it can
—  String literal: sequence of characters in quotes
—  Double quotes: “ abc3$#@(9”, “Hello world!”
—  Single quotes: ‘Hello world!’
—  This approximation results in representation error
—  When combined in expressions, the error can get worse
—  Example: try 0.1 + 0.2 to the Python prompt
—  Concatenation can only apply to strings
—  “ab” + “cd” evaluates to “abcd”
—  “ab” + 2 produces an error
—  “ab” + str(2) evaluates to “ab2”
13
Type: bool
Converting values between types
—  Type boolean or bool
—  Values: True, False
—  Operations: not, and, or
— 
— 
— 
14
—  Basic form: type(value)
—  float(2) converts value 2 to type float (value now 2.0)
—  int(3.9) converts value 3.9 to type int (value now 3)
—  Explicit conversion is also called “casting”
not b: True if b is false and False if b is true
a and b: True if both a and b are true; False otherwise
a or b: True if a is true or b is true; False otherwise
—  Narrow to wide: bool è int è float
—  Widening: Python does automatically if needed
—  Often come from comparing int or float values
—  Order comparison: i < j, i <= j, i >= j, i > j
—  Equality, inequality: i == j, i != j (note: = means something
different!)
— 
Example:
— 
— 
— 
1 / 2.0 evaluates to 0.5 (by casting 1 to float)
True + 3 evaluates to 4
False + 3 evaluates to 3
—  Narrowing: Python never does this automatically, we can force it though
— 
15
Example: float(int(3.9)) evaluates to 3.0
16
4
Operator precedence
Type: set of values and the operations on them
—  Type int:
—  Values: integers
—  Ops: +, -, *, /, %, **, …
—  What is the difference between the following?
—  2 * (1 + 3)
—  2 + 1 * 3
—  Type float:
—  Values: real numbers
—  Ops: +, -, *, /, **, …
—  Operations are performed in a set order
—  Parentheses make the order explicit
—  What happens when there are no parentheses?
—  There should be no ambiguity in terms of ordering of evaluation
—  Type bool:
—  Values;: True, False
—  Ops: not, and, or
—  Type: str:
—  Values: string literals with double quotes or single quotes
—  Ops: + (concatenation)
—  (Will see more types in the coming weeks)
—  Operator precedence: the fixed order Python processes operators in
absence of parentheses
17
Precedence of Python operators
—  Exponentiation: **
—  Unary operators: +, —  Binary arithmetic: *, /, %
18
Variables and assignments
—  A variable
—  Is a named memory location
—  Contains a value
—  Can be used in expressions
highest at the top
n  same line, same precedence
n  left-to-right among the same
n 
—  Binary arithmetic: +, -
—  Variables are created by assignment statements
—  ‘=‘ is the assignment operator
—  Examples:
b = 5
// assign 5 to b
h = 4.5
area = b * h
—  Comparisons: <, >, <=, >=
—  Equality relations: ==, !=
—  Logical not
—  Logical and
—  Logical or
19
b = 3
area = b * h
// repeated assignment into
// the same variable is acceptable
20
5
Dynamic typing
Dynamic typing (cont.)
—  Python is a dynamically typed language
—  A variable can hold values of any type
—  A variable can hold different types of values at different times
—  Use type(x) to find out the type of the value x at a given time
—  Use names of types for conversion, comparison
—  Often want to track the type in a variable
—  What is the result of evaluating x/y?
—  Depends on whether x, y are int or float values
—  Use the expression type(<expression>) to get type
—  type(2) evaluates to <type ‘int’>
—  type(x) evaluates to type of contents of x
—  The following is acceptable in Python
x=1
# x contains an int value 1
x = x / 2.0 # x now contains a float value 0.5
x = ‘apple’ # x now contains a str value ‘apple’
—  Can use in a boolean expression to test type
—  type(‘abc’) == str evaluates to True
—  a == b tests if a is equal to b
—  Alternative is a statically typed language (e.g., Java)
—  Each variable is restricted to values of just one type for its entire life
time
21
String: text as a value
Comments
—  string: a quoted sequence of characters
—  ‘abc d’ (Python prefers)
—  “abc d” (most other languages)
—  Comments are ignored by python interpreter
—  Used to add a note to your code for readability
—  Single line comments starts with a #
# This line is a comment
x = x + 1 # This is a partial line comment
—  How to write quotes in quotes?
—  Delineate with “other quote”
—  Examples
— 
— 
— 
— 
22
“‘“
‘“‘
“a’b’c” // third character is b
‘a”bb”c’
—  Multi-line comments with “”” … “””
“”” This is the first line of
a multi-line comment.
This is an example of a comment in three lines.
“””
23
24
6
Today’s extra
Do these before next class
—  If you are using a Mac, try “System Preferences” under the “Apple
symbol” at the top left corner of your screen
—  Finish reading Chapter 2
—  Start reading Chapter 3 of Downey
—  Try “Mission Control” and “Hot Corners” in particular
—  Do the Unix Lab if you are using a Mac
—  Do the DOS Lab if you are using a Windows machine
—  Next time
—  Functions (Chapter 3)
25
26
7