Python Notes – 7 : Files & directories

Welcome to our seventh note in our Python learning process. This note will talk specifically about files, directories, and exceptions.

Files

Opening a file creates a file object. Syntax is like that:

>>> f = open("test.dat","w")

>>> print f

<open file 'test.dat', mode 'w' at fe820>

The first parameter to open is the file name, the second parameter is the mode. Modes are: w for write, r for read.

To write data in the file we invoke the write method on the file object:

>>> f.write("Now is the time")

>>> f.write("to close the file")

After we done we can close the file like that:

>>> f.close()

The read method reads data from the file. With no arguments, it reads the entire contents of the file:

>>> text = f.read()

>>> print text

Now is the time to close the file

read can also take an argument that indicates how many characters to read.

If not enough characters are left in the file, read returns the remaining characters. When we get to the end of the file, read returns the empty string:

>>> print f.read(5)

Now i

>>> print f.read(1000006)

s the timeto close the file

>>> print f.read()

>>>

The write method write data to the file. It takes strings only.

>>> x = 52

>>> f.write (str(x))

Directories

When you create a new file by opening it and writing, the new file goes in the current directory (wherever you were when you ran the program). Similarly, when you open a file for reading, Python looks for it in the current directory. If you want to open a file somewhere else, you have to specify the path to the file, which is the name of the directory (or folder) where the file is located:

>>> f = open("/usr/share/dict/words","r")

>>> print f.readline()

Whatever exist

glob module

Returns filenames in a directory that match a pattern.

import glob

a = glob.glob("*.html")

b = glob.glob("image[0-5]*.gif")

Pattern matching is performed using rules of Unix shell. Tilde (~) and variable expansion is not performed.

fnmatch module

Matches filenames according to rules of Unix shell.

import fnmatch

if fnmatch(filename,"*.html"):

...

Case-sensitivity depends on the operating system.

Other File-Related Modules

• fcntl : Provides access to the fcntl() system call and file-locking operations

import fcntl, FCNTL

fcntl.flock(f.fileno(),FCNTL.LOCK_EX) # Lock a file

• tempfile : Creates temporary files

• gzip : Creates file objects with compression/decompression. Compatible with the GNU gzip program.

import gzip

f = gzip.open("foo","wb")

f.write(data)

Exceptions

Whenever a runtime error occurs, it creates an exception. Usually, the program stops and Python prints an error message.

>>> print 55/0

ZeroDivisionError: integer division or modulo

We can handle the exception using the try and except statements.

filename = raw_input('Enter a file name: ')

try:

f = open (filename, "r")

except:

print 'There is no file named', filename

The try statement executes the statements in the first block. If no exceptions occur, it ignores the except statement. If any exception occurs, it executes the statements in the except branch and then continues.

If your program detects an error condition, you can make it raise an exception.

def inputNumber () :

x = input ('Pick a number: ')

if x == 17 :

raise 'BadNumberError', '17 is a bad number'

return x

The raise statement takes two arguments: the exception type and specific

information about the error. And here how the previous example appears:

>>> inputNumber ()

Pick a number: 17

BadNumberError: 17 is a bad number

This note talked about files, directories, and exceptions. The next note will go into the object oriented features of Python.

Python Notes – 6 : Tuples & Dictionaries

Welcome to our sixth note in our Python learning process. In this note we will talk about tuples and dictionaries.

Tuples

Tuples are similar to lists but its elements can't be modified. Syntactically, a tuple is a comma-separated list of values:

>>> tuple = 'a', 'b', 'c', 'd', 'e'

Although it is not necessary, it is conventional to enclose tuples in parentheses:

>>> tuple = ('a', 'b', 'c', 'd', 'e')

To create a tuple with a single element, we have to include the ¯nal comma:

>>> t1 = ('a',)

>>> type(t1)

<type 'tuple'>

Without the comma, Python treats ('a') as a string in parentheses:

>>> t2 = ('a')

>>> type(t2)

<type 'string'>

Tuple assignment

Tuples assignment could be used to perform multiple assignments in one statement. For example, to swap a and b we can make it like that:

>>> a, b = b, a

The left side is a tuple of variables; the right side is a tuple of values. Each value is assigned to its respective variable.

Dictionaries

Dictionaries are like lists and tuples in that all of them are compound types.

Dictionaries are different than lists and tuples in that it can use any immutable type as an index not only integers like tuples and lists. Dictionaries are not ordered.

To create an empty dictionary and then add elements to it:

>>> eng2sp = {}

>>> eng2sp['one'] = 'uno'

>>> eng2sp['two'] = 'dos'

>>> print eng2sp

{'one': 'uno', 'two': 'dos'}

You can also create the dictionary by providing a list of key-value pairs:

>>> eng2sp = {'one': 'uno', 'two': 'dos', 'three': 'tres'}

>>> print eng2sp

{'one': 'uno', 'three': 'tres', 'two': 'dos'}

Dictionary Operations

del statement removes a key-value pair from a dictionary.

>>> inventory = {'apples': 430, 'bananas': 312, 'oranges': 525, 'pears': 217}

>>> print inventory

{'oranges': 525, 'apples': 430, 'pears': 217, 'bananas': 312}

>>> del inventory['pears']

>>> print inventory

{'oranges': 525, 'apples': 430, 'bananas': 312}

The len function also works on dictionaries; it returns the number of key-value pairs:

>>> len(inventory)

4

Dictionary methods

The keys method takes a dictionary and returns a list of the keys.

>>> eng2sp.keys()

['one', 'three', 'two']

The values method is similar; it returns a list of the values in the dictionary:

>>> eng2sp.values()

['uno', 'tres', 'dos']

The items method returns both, in the form of a list of tuples|one for each key-value pair:

>>> eng2sp.items()

[('one','uno'), ('three', 'tres'), ('two', 'dos')]

The method has_key takes a key and returns true (1) if the key appears in the dictionary:

>>> eng2sp.has_key('one')

1

>>> eng2sp.has_key('deux')

0

Aliasing and copying

Dictionaries are objects that can be aliased like that:

>>> opposites = {'up': 'down', 'right': 'wrong', 'true': 'false'}

>>> alias = opposites

Or copied to a new fresh copy like that:

>>> copy = opposites.copy()

This note completed the compound data types talk we started before with lists. Lists, tuples, and dictionaries are the compound data types that Python provides. You can use them as is or you can use them to build a more complex data types and containers.

Python Notes – 5 : Objects & Values

Welcome to our fifth note in our Python learning process. In this note we will talk about one of the core concepts of the Python language semantics .

Objects and Values

An object is something a variable can refer to. Every object has a unique identifier, which we can obtain with the id function.

>>> a = "banana"

>>> b = "banana"

>>> id(a)

135044008

>>> id(b)

135044008

Interestingly, lists behave differently. When we create two lists, we get two

objects:

>>> a = [1, 2, 3]

>>> b = [1, 2, 3]

>>> id(a)

135045528

>>> id(b)

135041704

Since both a and b refer to the same object, changes made to the object through one of them appear when you access the object through the other. This is called aliasing.

>>> a = [1, 2, 3]

>>> b = [1, 2, 3]

>>> b[0] = 5

>>> print a

[5, 2, 3]

This brief note is just to differentiate between values and objects. Objects not only the instances we create of classes, some language types treated as objects and others treated as values.

We will discuss object oriented in a separate note within days.

Python Notes – 4 : Lists

Welcome to our third note in our Python learning process. In this note we will talk mainly about lists, its functions and how to use it.

Lists - Creation

A list is an ordered set of values, where each value is identified by an index. The values that make up a list are called its elements. You could create it like:

>>> X = [12, 56, 87]

>>> print X

[12, 56, 87]

>>> Y = ["one", "three", "five"]

>>> print Y

["one", "three", "five"]

The list elements don't have to be the same type.

["hello", 2.0, 5, [10, 20]]

A list within another list is said to be nested.

Another way of creating special list of consecutive integers:

>>> range(1,5)

[1, 2, 3, 4]

If there is no start of the range, it will create starting from the 0

>>> range(10)

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

If there is a third argument, it specifies the space between successive values,

which is called the step size

>>> range(1, 10, 2)

[1, 3, 5, 7, 9]

Lists - Accessing elements

You access list elements by index. Indices starts with zero. Indices could be any integer expression.

>>> numbers = range(1,10)

>>> print numbers[0]

1

>>> print numbers[5-3]

3

Lists - Length

You can get the list length using len() method. Syntax like that:

>>> len(numbers)

9

Although a list can contain another list, the nested list still counts as a single

element. The length of this list is four:

['spam!', 1, ['Brie', 'Roquefort', 'Pol le Veq'], [1, 2, 3]]

Lists - operators

in is a boolean operator that tests membership in a sequence.

>>> horsemen = ['war', 'famine', 'pestilence', 'death']

>>> 'pestilence' in horsemen

1

>>> 'debauchery' in horsemen

0

Lists - for loops

The syntax is like:

for VARIABLE in LIST:

BODY

for horseman in horsemen:

print horseman

This is equivalent to:

i = 0

while i < len(LIST):

VARIABLE = LIST[i]

BODY

i = i + 1

Any list expression can be used in a for loop:

for number in range(20):

if number % 2 == 0:

print number

List - operations

The + operator concatenates lists:

>>> a = [1, 2, 3]

>>> b = [4, 5, 6]

>>> c = a + b

>>> print c

[1, 2, 3, 4, 5, 6]

Similarly, the * operator repeats a list a given number of times:

>>> [0] * 4

[0, 0, 0, 0]

>>> [1, 2, 3] * 3

[1, 2, 3, 1, 2, 3, 1, 2, 3]

The head function takes a list as a parameter and returns the first element.

>>> a = [1, 2, 3]

>>> head(a)

1

List - slices

>>> list = ['a', 'b', 'c', 'd', 'e', 'f']

>>> list[1:3]

['b', 'c']

>>> list[:4]

['a', 'b', 'c', 'd']

>>> list[3:]

['d', 'e', 'f']

>>> list[:]

['a', 'b', 'c', 'd', 'e', 'f']

List - deletion

del removes an element from a list:

>>> a = ['one', 'two', 'three']

>>> del a[1]

>>> a

['one', 'three']

You can use a slice as an index for del:

>>> list = ['a', 'b', 'c', 'd', 'e', 'f']

>>> del list[1:5]

>>> print list

['a', 'f']

• Strings are lists and can be treated the same way you treat lists.

Nested lists

A nested list is a list that appears as an element in another list. In this list, the

three-eth element is a nested list:

>>> list = ["hello", 2.0, 5, [10, 20]]

To extract an element from the nested list, we can proceed in two steps:

>>> elt = list[3]

>>> elt[0]

10

Or we can combine them:

>>> list[3][1]

20

In this note we talked mainly lists which are very powerful feature in Python. We will continue our learning in the upcoming notes.

Python Notes – 3 : Control Statements

Welcome to our third note in our Python learning process. In this note we will talk about conditional statements, iteration and keyboard input.

Conditional Statements

The normal flow of statements execution is sequential where the interpreter executes the statements in the same sequence that it appears in the script. If your program logic requires a different flow than the that, you should use conditional statements. The syntax looks like:

if x%2 == 0:

print x, "is even"

else:

print x, "is odd"

The statement indentation is the only statement scope identifier.

Conditional statements can be chained to whatever level you want. The syntax looks like:

if x < y:

print x, "is less than", y

elif x > y:

print x, "is greater than", y

else:

print x, "and", y, "are equal"

elif is an abbreviation of "else if."

Conditional statements can be nested to whatever level you want. Take care of indentation. The syntax looks like:

if x == y:

print x, "and", y, "are equal"

else:

if x < y:

print x, "is less than", y

else:

print x, "is greater than", y

Iteration

Iteration is the way to execute some statements multiple times. The syntax is like that:

While CONDITION:

STATEMENTS IN SCOPE

STATEMENTS IN SCOPE

STATEMENTS OUT OF WHILE SCOPE

The while will execute the in scope statements until the CONDITION evaluates to false. Example:

X = 10

While X != 0:

Print X

X = X - 1

print "Finish"

This will print numbers from 10 to 1 then "Finish".

Keyboard input

Python provides built-in functions that get input from the keyboard. The simplest is called raw input. When this function is called, the program stops and waits for the user to type something. When the user presses Return or the Enter key, the program resumes and raw input returns what the user typed as

a string:

>>> input = raw_input ()

What are you waiting for?

>>> print input

What are you waiting for?

In this note we talked about conditional statements, iteration and keyboard input. These are another way of organizing your program logic to satisfy accomplish your program goal. We will continue our learning in the upcoming notes.

Python Notes – 2 : Variables, Statements, Expressions, Operators, and Functions

Welcome to our second note in our Python learning process. In this note we will talk about variables, statements, expressions, operators, comments, and functions. These are the very basic building blocks of you programs whatever its final size

Variables

The assignment statement creates new variables and gives them values

>>> message = "What's up, Doc?"

>>> n = 17

>>> pi = 3.14159

Python is a dynamically typed language. Which means that variables' types don't have to be defined before the variables use. The python interpreter figure out what type a variable is when you first assign it a value.

Variables naming

• Variable name can contain letters, numbers, and underscore.
• Variable name must begin with letter.
• Variable names are case-sensitive ( x different than X ).
• Variable names can't be one of the Python language reserved words, which are :

and	continue	else	for	import	not	raise
assert	def	except	from	in	or	return
break	def	exec	global	is	pass	try
class	elif	finally	if	lambda	print	while

Statements

A statement is an instruction that the Python interpreter can execute. A Python script is a sequence of statements. The results appear one at a time as the statements execute.

For example, the script

>>> print 1

>>> 1

>>> x = 2

>>> print x

>>> 2

Expressions

An expression is a combination of values, variables, and operators. If you type an expression on the command line, the interpreter evaluates it and displays the result:

>>> 1 + 1

2

Boolean Expressions

A boolean expression is an expression that is either true or false. Boolean expressions are expressions that uses logical operators. There are three logical operators: and, or, not. The operands of logical operands should be boolean expressions. In Python there is no boolean data type instead 0, '', [], (), {}, and None are false in a boolean context; everything else is true. The absence of boolean data types results in the following behavior of logical operators:

• All the behaviors depends on the order of expressions evaluation, which is from left to right.
• and : returns the first false value, if all values are true, returns the last value.

>>> 'a' and 'b'

'b'

>>> ' ' and 'b'

' '

>>> 'a' and 'b' and 'c'

'c'

• or : returns the first true value, if all values are false, returns the last value

>>> 'a' or 'b'

'a'

>>> ' ' or 'b'

'b'

>>> ' ' or [] or {}

{}

• not : evaluates the boolean expression that follows it and inverts it whole value.

>>> not (1 and 0)

True

>>> not (1 or 0)

False

Operators and operands

Operators are special symbols that represent computations like addition and multiplication. The values the operator uses are called operands.

The symbols +, -, and /, and the use of parenthesis for grouping, mean in

Python what they mean in mathematics. The asterisk (*) is the symbol for multiplication, and ** is the symbol for exponentiation.

Order of operations

When more than one operator appears in an expression, the order of evaluation depends on the rules of precedence. The following is operators from highest precedence to lower precedence:

• Parentheses
• Exponentiation
• Multiplication / Division
• Addition / Subtraction

Operators with the same precedence are evaluated from left to right.

Comments

Comments start with #.

Built-in Functions

Here are some of the built-in functions available in Python:

• Type conversion functions which converts values from one type to another. Like:
  • int() :

>>> int("32")

32

>>> int("Hello")

ValueError: invalid literal for int(): Hello

• float():

>>> float(32)

32.0

>>> float("3.14159")

3.14159

• str():

>>> str(32)

'32'

>>> str(3.14149)

'3.14149'

• Math functions which provides most of the familiar mathematical functions. These functions are implemented in a module called math, we will talk about modules soon. But for now, if you want to use these functions, you have to import the math module. This is very straight foreword, using following statement
  • >>> import math
  • After the import statement, you can simply call the math functions.

>>> decibel = math.log10 (17.0)

>>> angle = 1.5

>>> height = math.sin(angle)

>>> degrees = 45

>>> angle = degrees * 2 * math.pi / 360.0

>>> math.sin(angle)

0.707106781187

>>> math.sqrt(2) / 2.0

0.707106781187

There are many other built-in functions, we mentioned these two types just as examples.

Creating your own functions

A function is a named sequence of statements that performs a desired operation. This operation is specified in a function definition.

The syntax for a function definition is:

def FUNCTION_NAME( LIST OF PARAMETERS ):

STATEMENTS

• def is a keyword.
• FUNCTION_NAME could be anything except Python keywords.
• LIST OF PARAMETERS are the ordered list parameters the function needs to operate. The empty parentheses indicate no parameters needed.
• The function body doesn't have starting or ending characters (like "{" and "}" in C like languages ). Indentation is the only scope identifier for statements in Python. The first statement that doesn't follow the indentation, will be considered out of the function and will mark the function termination.

# just to show the function scope and structure

def sample_func():

    Print "we are in the function body scope"

    Print "we still in the function scope"

Print "we are out of the function scope"

# another function with parameters

def sum(x, y):

    print x + y

• To call a function just use the function name followed by the list of parameters between parentheses

>>> sample_func()

>>> sum(1,2)

3

• Functions can call other functions in their body.
• Functions can return values to its caller

def sum(x,y):

return x +y

def print_sum(x,y):

print sum(x,y)

print_sum(2,3)

5

• When you create a local variable inside a function, it only exists inside the function, and you cannot use it outside. The following will generate a run-time error when trying to access Z variable.

def sum(x,y):

Z = x +y

print Z # will generate a runtime error

Returning multiple values

def divide(a,b):

q = a/b

r = a - q*b

return q,r

>>> x,y = divide(42,5) # x = 8, y = 2

>>> x

8

>>> y

2

In this note we talked about variables, statements, expressions, operators, comments, and functions. These are the very basic building blocks of you programs whatever its final size. We will continue our learning in the upcoming notes.

Python Notes – 1 : Setup

I write this notes about Python during my learning process of it. I'm not an expert in Python. I'm just writing this notes while I learn Python aiming to help others who are learning it too. Never hesitate to comment on anything you want.

Origins of Python

• Developed by Guido van Rossum.
• Derived from ABC, a teaching language that was developed in 1980s.

Why Pyhton ?

• Python code is pretty simple, compact and easy to learn.
• Code simplicity let you focus on the core program functionality.
• Suitable for programming languages introductory courses.
• Provides a balance between the practical and the conceptual. You start writing after the first tutorial; and if you want to go further in advanced topics you will find a large library of modules that can be used to do all sorts of tasks ranging from web-programming to graphics.
• Python borrows features from both functional programming languages and object-oriented programming languages, which enables it to serve as an excellent foundation for introducing important computer science concepts.
• Python allow you to see higher level of success and a lower level of frustration :)

How to setup Python ?

Python is free downloadable from many source, and it have also many IDEs and editors. Aiming to start coding quickly without wasting time in IDE exploration, setup, configuration problems; we will use just the ActivePython. Let's see how to do that:

1. Download ActivePython, it is free.
2. After you install ActivePython you will got that the following change in your start menu. We will not use Python Intercative Shell nor PythonWin Editor in our first steps, we will use them in upcoming notes.

     

1. In our first note we will use Python from the Windows Command window. To do that we need to add the Python installation path to the Environment Variables of the operating system.
2. If you are using Windows XP or Windows 2000 do the following (if you are using Windows Vista go for step 5):
   1. Right-click My Computer and select Properties from the displayed context menu. You will got the System Properties dialog

     

1. Then click the Advanced Tab

     

1. Then click the Environment Variables button near the left bottom. You will got the Environment variables dialog

     

1. Select the variable path from the second list called System Variables. Then click Edit button, you will got the Edit System Variable dialog box

     

1. Append the Python installation path to the string in Variable value textbox. The default installation path for ActivePython in our case is C:\Python25\; the semicolon ';' is not part of the path but is mandatory to be added at the end. This semicolon acts as the delimiter that enables the operating system to differentiate between this list of paths.

1. If you using Windows Vista, do the following (very similar steps):
   1. Right-click My Computer and select Properties from the displayed context menu. You will got the System Properties dialog

     

1. Then the link Advanced system settings on the left menu, you will got the System Properties dialog box

     

1. Then click the Advanced tab

     

1. Click the Environment Variables button near the right bottom, you will got Environment Variables dialog box

     

1. Select the variable path from the second list called System Variables. Then click Edit button, you will got the Edit System Variable dialog box

     

1. Append the Python installation path to the string in Variable value textbox. The default installation path for ActivePython in our case is C:\Python25\; the semicolon ';' is not part of the path but is mandatory to be added at the end. This semicolon acts as the delimiter that enables the operating system to differentiate between this list of paths.

Now, we have Python installed and configured on our machines.

Let's begin in Python

Python is a high-level programming language. It is interpreted language, which means that its programs are executed by an interpreter rather than a compiler. There are two ways to use the interpreter:

• Command-line mode where you type your python programs at the command-line and the interpreter prints the result.
  • Open the windows command window and type python, to invoke the python interpreter.
  • Type print "Hello world !" and press enter. Guess what, you have wrote your first program in Python :)

      

• Script mode where you write your program in a file and use the interpreter to execute the contents of the file. Such a file called script. By convention python files have extension .py.
  • To try that mode, create a file script1.py in your Python installation folder ( default is : C:\Python25 ).
  • In this file write print "Hello world !" , save and close.
  • Open the windows command window and type python, to invoke the python interpreter.
  • Type import script1 and press enter. You just ran your first Python program.

      

We will use the script mode in most of our notes.

This is just the beginning of our notes about Python. Now we have Python installed and configured on our machines. We will continue our learning in the upcoming notes.