Elements CS313E: of Software Design
Basic Python Dr. Bill Young Department of Computer Sciences University of Texas at Austin
Last updated: September 3, 2010 at 08:28
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Acknowledgement
Much of the material in this introductory slideset if from Dr. Shyamal Mitra, and used with his blessing.
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Introduction to Python
Python is a simple but powerful scripting language. It was developed by Guido van Rossum in the Netherlands in the late 1980s. The language takes its name from the British comedy series Monty Python's Flying Circus. Python is an interpreted language unlike C or Fortran that are compiled languages. Clean syntax that is concise. You can say a lot more with fewer words. Design is compact. You can carry all the language constructs in your head. There is a very powerful library of useful funcions available. This means that you can be productive quite easily and fast. You will be spending more time solving problems and writing code than grappling with the idiosyncrasies of the language.
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Running Python: Interactively
There are several ways to run Python. >>> print "Hello World!" Hello World! This uses the read-execute-print loop.
1 2 3 4
Read in a legal Python form. Execute the form. Print the result to standard output. Go to step 1.
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Running Python: From File
For longer programs you can store your program in a file that ends with a .py extension. Example: save in a file called Hello.py is the single statement: print "Hello World!" To run your program, type at the command line > python Hello.py You can also create a stand alone script. On a Unix / Linux machine you can create a script called Hello.py containing the following lines (assuming that's where your Python implementation lives): #!/lusr/bin/python print "Hello World!"
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Running Python: From File
Let's try running this: > Hello.py Hello.py: Permission denied. What went wrong? Before you run it, you must make it into an executable file: > chmod +x Hello.py Then run it: > Hello.py Hello World!
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Structure of a Computer Language
Character set Variables Types Operators Assignments Conditionals Loops Functions Arrays Input / Output
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Basic Syntax: Character Set
Python uses the traditional ASCII character set. Recent versions also recognize the Unicode character set. The ASCII character set is a subset of the Unicode character set. Python does not have a separate character type. A character is represented as a string containing one item (e.g., "a").
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Identifiers
Python is case sensitive. These are rules for creating an indentifier: Must start with a letter or underscore ( ) Can be followed by any number of letters, digits, or underscores. Cannot be a reserved word. Python reserved words are: False, None, True, and, as, assert, break, class, continue, def, del, elif, else, except, finally, for, from, global, if, import, in, is, lambda, nonlocal, not, or, pass, raise, return, try, while, with, yield.
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Variables
A variable in Python denotes a memory location. In that location is the address where the value is stored in memory. Consider this assignment: x = 1 A memory location is set aside for the variable x. The value 1 is stored in another place in memory. The address of that location is stored in the memory location denoted by x. This simple observation explains a lot about Python.
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Variables and Values
The values stored in variables are pointers (addresses). That's why Python variables are untyped. That's why it's OK to assign a value of "another type" to x. If you later re-assign x: x = 2.75 the value 2.75 is stored in another memory location; its address is placed in the memory location denoted by x and the memory occupied by the value 1 may be reclaimed by the garbage collector.
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Types in Python
Is it correct to say that there are no types in Python? Yes and no. It is best to say that Python is "dynamically typesd." Variables in Python are untyped, but values have associated types which may be primitive types or classes. In some cases, you can convert one type to another. Most programming languages assign types to both variables and values. This has its advantages and disadvantages. Can you guess what the advantages are?
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Types in Python
Type str Description An immutable sequence of characters. In Python 2, strings are a sequence of characters. Unicode strings need to be declared by prefixing with the letter u. In Python 3 strings are Unicode by default. An immutable sequence of bytes Mutable, can contain mixed types Immutable, can contain mixed types Unordered, contains no duplicates A mutable group of key and value pairs An immutable fixed precision number of unlimited magnitude An immutable floating point number (system-defined precision) An immutable complex number with real number and imaginary parts An immutable truth value Syntax example 'Wikipedia' "Wikipedia" """Spanning multiple lines""" b'Some ASCII' b"Some ASCII" [4.0, 'string', True] (4.0, 'string', True) {4.0, 'string', True} {'key1': 1.0, 3: False} 42 3.1415927 3+2.7j True, False
bytes list tuple set dict int float complex bool
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Operators: Arithmetic Operators
These are the arithmetic operators that operate on numbers (integers or floats). The result of applying an arithmetic operator is a number. Addition: + Subtraction: - Multiplication: Division: / Remainder: % Exponentiation:
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Relational Operators
Python has 6 comparison operators. The result of applying the comparison operators is a Boolean: True or False. Equal to: == Not equal to: != Greater than: > Greater than or equal to: >= Less than: < Less than or equal to: <= You may apply the comparison operator between two operands like so (a > b). You can also apply the comparison operators in sequence
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Boolean Operators
Python has two Boolean literals: True and False. But Python will also regard any of the following as False in a Boolean context: the number zero (0), the empty string (""), the reserved word None. All other values are interpreted as True. There are 3 Boolean operators: not: unary operator that returns True if the operand is False and vice versa. x and y: if x is False, then that value is returned; otherwise y is evaluated and the resulting value is returned. x or y: if x is true, then that value is returned; otherwise y is and evaluated the resulting value is returned.
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Assignments
A variable can be assigned the value of a literal or to an expression. a b c d e f = = = = = = 1 2.3 False 5L 8 + 6j "Hello" # # # # # # 1 is an integer literal 2.3 is a floating point literal False is a boolean literal 5L is a long integer literal 8 + 6j is a complex literal "Hello" is a string literal
Recall what actually goes into the variable! What do you think happens in each of these cases? An expression is composed of variables and operators. An expression is evaluated before its value used. Python allows for multiple assignments at once. x, y = y, x
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Conditionals
In Python, a conditional allows you to make a decision. You can use the keyword elif that is a contraction of the words else and if. if (cond1): ... elif (cond2): ... else: ... The body of if, elif, and else parts need to be indented.
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Loops
The loop constructs of Python allow you to repeat a body of code several times. There are two types of loops: definite loops and indefinite loops. You use a definite loop (for loop) when you know a priori how many times you will be executing the body of the loop. You use an indefinite loop (while loop) when you do not know a priori how many times you will be executing the body of the loop. To write an efficient loop structure you must ask yourself these questions: Where do I start? When do I end? How do I go from one iteration of the loop to the next?
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While Loops
Syntactically the simplest loop construct is the while loop. while (cond): ... loop_body ... What is the semantics of this construct? I.e., what does this really do?
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For Loops
A definite loop requires a loop counter which takes on a series of values. Example: write a loop that executes the body 10 times. for i in range (10): ... loop_body ... The loop counter i goes through the values 0 through 9 as the loop iterates. We can also specify the beginning and ending value of i. for i in range (3, 15): ... loop_body ... What values does i take on in this loop?
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For Loops
We can also specify the step size for the loop counter. for i in range (1, 10, 2): ... loop_body ... What values will i take on here? How would you process the sequence backwards? You can also enumerate a sequence of values for the loop counter. for i in [7, 4, 18, 12]: ... loop_body ...
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Functions
A function is an encapsulation of some functionality. Functions can be called at the top level or from within another function (or itself). A function may or may not return a value or values. The structure of a function definition is as follows: def function-name ([formal-parameters]): ... body-of-the-function ... The function-name is an identifier. You can have zero or more formal parameters, but must have the parentheses.
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Functions
The formal parameters are placeholders that accept values sent to it from the calling function. When a function is called, the parameters that are passed to it are called actual parameters. For a complete list of built-in functions see: docs.python.org/library/functions.html.
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Function Examples
def addTwo (a, b): return a + b def divide (a, b): return a / b, a % b def isEven (x): return (x % 2 == 0) def gcd (m, n): while (m != n): if (m > n): m = m - n else: n = n - m return m
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Function Examples (2)
def coPrime (a, b): if (gcd(a, b) != 1): return else: print "%0d and %0d are co-prime" % (a, b) def main(): x = 2 y = 3 z = addTwo (x, y) p, q = divide (x, y) if (isEven(z)): print z coPrime (x, y) main()
What does this program do?
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User-Defined Functions
To call a user defined function that is in the same program you simply call it by name without using the dot operator. If the function does not return a value, then call it on a line by itself. If it returns values then assign those return values to variables.
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Import Modules
There are other functions that reside in modules that have to be loaded before you can use them: string, math, random, etc. You load these modules by using the import directive. import string import math, random After you load the module you can call on the functions using the dot operator. x = 7 y = math.sqrt (x) z = random.random()
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Lists in Python
The analogue of an array in Python is a list. A list is heterogenous and dynamic. What does that mean? The size is not specified at creation and can grow and shrink as needed. Python provides built-in functions to manipulate a list and its contents. There are several ways in which to create a list: enumerate all the elements create an empty list and then append or insert items into the list.
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Lists in Python
# Enumerate the items a = [1, 2, 3] # Create an empty list and append or insert a = [] a.append(1) # a = [1] a.append(2) # a = [1, 2] a.insert(1, 3) # a = [1, 3, 2] # Create a two dimensional list b = [ [1, 2, 3], [4, 5, 6], [7, 8, 9] ]
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Lists in Python (3)
To obtain the length of a list you can use the len() function. a = [1, 2, 3] length = len (a) # length = 3
The items in a list are indexed starting at 0 and ending at index length - 1. You can also slice a list by specifying the starting index and the ending index and Python will return to you a sub-list from the starting index and upto but not including the end index. a = [1, 9, 2, 8, 3, 7, 4, 6, 5] b = a[2:5] # b = [2, 8, 3]
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Lists in Python (4)
One of the most important operations that you can do with a list is to traverse it, i.e. visit each element in the list in order. There are several ways in which to do so: a = [9, 2, 6, 4, 7] for item in a: print item,
# 9 2 6 4 7
Other useful functions to sort, reverse, etc. lists are here: www.python.org/doc/2.5.2/lib/typesseq-mutable.html.
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Input and Output
You can prompt the user to enter a number by using function input() or a string by using the function raw input(). The function then waits for the user to enter the value and reads what has been typed only after the user has typed the Enter or Return key. n = input ("Enter a number: ") name = raw_input ("Enter your name: ") The print command allows you to print out the value of a variable. If you want to add text to the output, then that text has to be in single or double quotes. The comma (,) is used as the concatentation operator. print 'n = ', n print "name = ", name
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