Tips and Examples

• Studying for the Midterm with Flashcards

Review

• Dictionaries
• Dictionary Literals
• Creating an Empty Dictionary
• Getting Values from a Dictionary
• Adding Elements to a Dictionary
• Changing a Dictionary Value
• Looping Through a Dictionary
• When To Use a Dictionary
• Lists versus Dictionaries
• The in And not in Operators
• Deleting Elements from A Dictionary
• Getting the Number of Elements in a Dictionary
• Sets in Mathematics
• Set Membership
• Subsets and Supersets
• Union of Sets
• Intersection of Sets
• Difference between Sets
• Sets in Python
• Creating a Set in Python
• Set Literals
• The Empty Set
• Adding Elements to a Set
• Removing Elements from a Set
• The Size of a Set
• When Are Sets Equal?
• for Loops with Sets
• Testing for Set Membership
• Union of Sets in Python
• Intersection of Sets in Python
• Difference between Sets in Python
• Subsets and Supersets in Python
• min And max with Sets
• The os Module
• os.getcwd()
• os.listdir(path)
• os.chdir(path)
• Running Unix Commands within Python
• os.environ
• The os.path Module
• os.path.isfile(path) and os.path.isdir(path)
• os.path.basename(path)
• The sys Module
• Getting Values from the Command Line
• Leaving a Running Script
• Usage Messages
• The Characters in Regular Expressions
• Ordinary Characters in Regular Expressions
• Meta-characters in Regular Expressions
• The . Meta-character
• The * Meta-character
• The + Meta-character
• The ? Meta-character
• The \ Meta-character
• Character Classes
• \d and \D Character Classes
• The \w and \W Character Classes
• The \s and \S Character Classes
• Getting Strings from a Match
• The ( ) Meta-characters
• Repetition in Regular Expressions
• Specifying a Range of Repeating Characters
• Creating Custom Character Classes
• Ranges of Characters in a Character Class
• Greedy versus Non-greedy Matching

Homework 7

I have posted homework 7 here.

It is NOT due this coming Sunday.

Instead it is due Sunday, March 24th.

This is to give you time to study for the midterm.

And to give me time to score it.

Midterm

The Midterm exam for this course will be held on Tuesday, March 19th.

That is the first Tuesday after the Spring Break.

The exam will be given in this room.

It will consist of questions like those on the quizzes along with questions asking you to write short segments of Python code.

60% of the points on this exam will consist of questions from the Ungraded Class Quizzes.

There will be 15 of these questions worth 4 points each.

The other 40% of points will come from four questions that ask you to write a short segment of code.

Each of the code questions is worth 10 points each.

To study for the code questions you should know

• Dictionaries
• Sets
• How to use the os and sys modules
• How to write a regular expression

A good way to study for the code questions is to review the Class Exercises and homework solutions.

Today's class will be a review session.

You will only be responsible for the material in the Class Notes for today's class on the exam.

You will find the Midterm review Class Notes here.

If for some reason you cannot take the exam on the date mentioned above you must contact me to make alternate arrangements.

The Midterm is given on paper.

I scan each exam paper and upload the scans to Gradescope.

I score the exam on Gradescope.

You will get an email from Gradescope with your score when I am done.

The Midterm is a closed book exam.

You are not allowed to any resource, other than what is in your head, while taking the exam.

Cheating on the exam will result in a score of 0 and will be reported to the Administration.

Remember your Oath of Honesty.

To prevent cheating, certain rules will be enforced during the exam.

Quiz 5

Let's look at the answers to Quiz 5

No Class Exercise or Class Quiz

Today is a review session.

There will no Class Exercise or Class Quiz today.

Questions

Are there any questions before I begin?

Tips and Examples

Studying for the Midterm with Flashcards

• 60% of the point on the Midterm come from Class Quiz Questions
• You can use the flashcards you create for these questions when studying for the Midterm
• But not all Class Quiz Questions will appear on the Midterm
• If the flashcard question covers a topic not in the Midterm Review ...
• you do not have to study it for the exam
• You should remove these flashcards from your collection

Review

Dictionaries

• Dictionaries have multiple entries
• Each entry has two parts, a key and a value
• You use the key to get the value
• So an entry in a Python dictionary is a key-value pair
• You cannot have a dictionary entry that is a key with no value ...
• or a value with no key

Dictionary Literals

• A literal is a value written directly inside the code
• Dictionary literals contain a number of entries separated by commas ...
• and enclosed in curly braces, { }
• The key and value are separated by a colon, :

  >>> digit_names = {1 : "one", 2 : "two", 3 : "three"} 
  >>> digit_names
  {1: 'one', 2: 'two', 3: 'three'} 

Creating an Empty Dictionary

• An empty dictionary is a dictionary with no entries
• You created and empty dictionary like this

  new_dict = {}

Getting Values from a Dictionary

• To get the value of associated with a key you use the [ ] operator
• You place this operator after the variable pointing to the dictionary ...
• and you put the key inside the [ ]
• So if I have the following dictionary

  >>> students = {"023413" : "Alan Smith", "01234" : "John Doe"}

• I can get the name of the student with ID 01234 like this

  >>> students["01234"]
  'John Doe'

Adding Elements to a Dictionary

• Dictionaries have no methods to add entries to them
• Instead, you use the [ ] to assign a value to a new key
• This creates a new entry in the dictionary

  >>> email_addresses
  {}
  >>> email_addresses["joe"] = "joe@gmail.com"
  >>> email_addresses
  {'joe': 'joe@gmail.com'}

• The entry will only be added ...
• if the key is not already used in the dictionary
• If the key is already in the dictionary ...
• it will change the value associated with that key

  >>> email_addresses
  {'joe': 'joe@gmail.com'}
  >>> email_addresses["joe"] = "bigmanjoe@hotmail.com"
  >>> email_addresses
  {'joe': 'bigmanjoe@hotmail.com'}

Changing a Dictionary Value

• The way you change a value in a dictionary is similar to what you do in a list
• You use an assignment statement with the [ ] operator
• The left hand side of the assignment statement ...
• is the dictionary variable followed by [ ] ...
• with a key inside
• The new value appears on the right hand side

  >>> students
  {'01234': 'John Doe', '023413': 'Alan Smith'
  >>> students["023413"] = "Al Smith"
  >>> students
  {'01234': 'John Doe', '023413': 'Al Smith'}

• This means that when you see a statement like this

  email_addresses["joe"] = "joe@gmail.com"

• You cannot tell by looking at it whether you have added an entry ...
• or changed the value associated with an exiting entry
• The key determines what happens
• If the key is not already in the dictionary ...
• a new entry has been added
• If the key is already in the dictionary ...
• an existing entry gets a new value

Looping Through a Dictionary

• You can loop through a dictionary using a for loop
• The loop variable gets the values of each key ...
• as it loops through the statements in the code block
• You can use the key to get the value

  >>> scores = {"amy" : 100, "bill": 95, "dave" : 60, "sally" : 95}
  >>> for name in scores:
  ...     print(name, scores[name])
  ... 
  dave 60
  amy 100
  sally 95
  bill 95

• You can print the keys in sorted order using the sorted function
• sorted is a built-in function ...
• that takes anything you can use in a for loop ...
• and returns a sorted list
• If you run sorted on a dictionary it will return a list of sorted keys

  >>> for name in sorted(scores):
  ...     print(name, scores[name])
  ... 
  amy 100
  bill 95
  dave 60
  sally 95 
  

When To Use a Dictionary

• Lists and tuples are used when the elements are not particularly unique
• In a list of quiz scores there is nothing special about an individual score
• But every student is different
• Each student has a unique identity
• Dictionaries are used to store data about unique things

Lists versus Dictionaries

• Both lists and dictionaries are objects that hold multiple values
• In a list you access a value by its index
• In a dictionary you access a value by its key
• You can think of a list as a collection of values
• While a dictionary is a collection of variables
• A variable is a place in memory with a name ...
• that holds a value

The in And not in Operators

• The in and not in operators work with dictionaries
• The same way they work in sequences
• It tells whether the dictionary contains a key

  >>> digit_names = {"one":1, "two":2, "three":3, "four":4, "five":5 }
  >>> "one" in digit_names
  True
  >>> 1 in digit_names
  False

• The not in operator returns True if the first operand is not a key in the dictionary

  >>> "one" not  in digit_names
  False
  >>> 1 not in digit_names
  True
  

Deleting Elements from A Dictionary

• To delete an entry in a dictionary you use a delete statement
• The general format of such a statement is

  del DICTIONARY_NAME[KEY] 

• Where DICTIONARY_NAME is a variable that points to a dictionary object
• And KEY is the key of the entry to be deleted

  >>> words_integers
  {'three': 3, 'five': 5, 'two': 2, 'one': 1, 'four': 4}
  >>> del words_integers["five"]
  >>> words_integers
  {"three": 3, "two": 2, "one": 1, "four": 4}

• If you use a key that does not exist
• You will get a KeyError exception is raised

  del words_integers["six"]
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  KeyError: 'six'

Getting the Number of Elements in a Dictionary

• The len function returns the number of entries in a dictionary

  >>> email_addresses
  {'Chris': 'chrisk@yahoo.com', 'Alan': 'alanh@gmail.com'}
  >>> len(email_addresses)
  2
  >>> words_integers
  {'five': 5, 'one': 1, 'six': 6, 'two': 2, 'four': 4, 'three': 3}
  >>> len(words_integers)
  6

Attendance

Sets in Mathematics

• A set is an unordered collection of distinct objects
• You can put anything into a set
• But you can't add a value that is already there
• If you try, the set will not change
• The set with nothing in it is called the empty set

Set Membership

• If the value x is in set A we say that x is a member of A

Subsets and Supersets

• If you have two sets, A and B ...
• and all the values in A are also in B
• Then A is a subset of B
• Another way of describing this situation is that B is a superset of A
• The situation is shown in the following diagram

Union of Sets

• Again start with two sets A and B
• The set of all the elements A and all the elements of B is called the union of A and B
• In the diagram below the union of A and B is shown in red

Intersection of Sets

• The set of elements which are member of A ...
• and also members of B ..
• is the intersection of A and B
• In the diagram below, the intersection of A and B is shown in red

Difference between Sets

• The set of all elements of A not in B is the difference between A and B
• In the diagram below, the difference between A and B is shown in red

Sets in Python

• A set in Python is an object that holds an unordered collection of unique items
• The items inside a set can by of any data type ...
• as long as the data type is immutable

Creating a Set in Python

• You create a set in Python using the built-in set function
• set takes a single argument
• That argument must be iterable
• A Python object is iterable if you can use it in a for loop
• Here is an example

  >>> num_list = [1,2,3]
  >>> num_set  = set(num_list)
  >>> num_set
  {1, 2, 3}

Set Literals

• A list literal is has values, separated by commas, inside square brackets

  >>> list_1 = [1, 2, 3, 4, 5]
  >>> type(list_1)
  <class 'list'>

• A set literal uses curly braces

  >>> nonsense = {"foo", "bar", "bletch"}
  >>> type(nonsense)
  <class 'set'>

The Empty Set

• We can use empty square brackets to create an empty list

  >>> empty = []
  >>> type(empty)
  <class 'list'>

• But we cannot use empty curly braces to create a empty set
• The empty curly braces are an empty dictionary

  >>> empty = {}
  >>> type(empty)
  <class 'dict'>

• When the creators of Python got to set literals
• They ran out of symbols to enclose the elements
• So they had to reuse { }
• So how do you create an empty set?
• You run set with no arguments

  >>> set_1 = set()
  >>> set_1
  set()

• That is why an empty set

  empty_set = set()

  looks like this

  >>> empty_set
  set()

Adding Elements to a Set

• Sets are mutable objects so they can be changed at any time
• The add method adds a single element to a set
• So if we start with an empty set

  >>> s1 = set()
  >>> s1
  set()

• We can use add to add individual elements

  >>> s1.add(1)
  >>> s1
  {1}

• If you add an element that is already in the set
• Nothing will change

  >>> s1.add(1)
  >>> s1
  {(3, 3, 3), 1, 'two'}

• But it won't raise an exception

Removing Elements from a Set

• To remove an element from a set use one of two methods
  • discard()
  • remove()
• Both methods take a single argument
• The value that is to be removed

  >>> numb_set
  {1, 2, 3, 4, 5}
  >>> numb_set.discard(2)
  >>> numb_set
  {1, 3, 4, 5}
  >>> numb_set.remove(4)
  >>> numb_set
  {1, 3, 5}

• The only difference is what happens when you remove a value that is not in the set
• discard will say nothing

  >>> numb_set.discard(2)
  >>> numb_set
  {1, 3, 5}

• But remove will raise an exception

  >>> numb_set.remove(4)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  KeyError: 4

The Size of a Set

• The len function gives the size of a set

  >>> set_1 = {1, 2, 3}
  >>> len(set_1)
  3
  >>> set_2 = {3, 2, 1}
  >>> len(set_2)
  3
  >>> set_3 = {"one", "two", "three", "four"}
  >>> len(set_3)
  4

When Are Sets Equal?

• If two sets have the same elements
• They are equal

  >>> set_1 = {1, 2, 3}
  >>> set_2 = {3, 2, 1}
  >>> set_1 == set_2
  True

for Loops with Sets

• Sets are iterable
• This means that they can be used in a for loop
• The general format of a for loop looks like this

  for LOOP_VARIABLE in ITERABLE_OBJECT:
      STATEMENT
      ...

• If you use a set in a for loop you will get each element in the set

  >>> names = {"amy", "bill", "dave", "sally"}
  >>> for name in names:
  ...     print(name)
  ... 
  1
  2
  3
  4
  5

• To print the elements in sorted order use the sorted function

  >>> >>> for name in names:
  ...     print(name)
  ... 
  dave
  amy
  sally
  bill

Testing for Set Membership

• The in operator tells you if a value is contained in a set

  >>> set_1
  {1, 2, 3, 4, 5}
  >>> 7 in set_1
  False
  >>> 8 in set_1
  False
  >>> 3 in set_1
  True

• The not in operator tells you if an element is not in a set

  >>> 8 not in set_1
  True
  >>> 3 not in set_1
  False

Union of Sets in Python

• We can form the union of two sets in Python by using the union method

  >>> A = {1, 4, 8, 12}
  >>> B = {1, 2, 6, 8}
  >>>  A.union(B)
  {1, 2, 4, 6, 8, 12}

• The union operation is symmetrical
• This means that

  A.union(B)

  is the same as

  B.union(A)

Intersection of Sets in Python

• Set objects in Python have an intersection method

  >>> A
  {8, 1, 12, 4}
  >>> B
  {8, 1, 2, 6}
  >>> A.intersection(B)
  {8, 1}

• Intersection is also symmetrical so

  A.intersection(B)

  is the same as

  B.intersection(A)

Difference between Sets in Python

• In Python, we can use the set difference method

  >>> A
  {8, 1, 12, 4}
  >>> B
  {8, 1, 2, 6}
  A.difference(B)
  {12, 4}

• Set difference is not a symmetric operation

Subsets and Supersets in Python

• We can tell if one set is a subset of another using the issubset method
• If we have two sets

  >>> A = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
  >>> B = {1, 3, 5, 7, 9}

• We can ask if one set is the subset of another like this

  >>> A.issubset(B)
  False
  >>> B.issubset(A)
  True

• We can ask if one set is a superset of another using the issuperset method

  >>> A.issuperset(B)
  True
  >>> B.issuperset(A)
  False
  

min And max with Sets

• To find the set element with the maximum value ...
• you can use the max built-in function

  >>> B = {1, 3, 5, 7, 9}
  >>> max(B)
  9
  

• To find the set element with the minimum value ...
• use the min function

  >>> min(B)
  1
  

The os Module

• The os module allows you do naything the operating system can do

os.getcwd()

• os.getcwd() returns a string with the pathname of your current directory

  >>> os.getcwd()
  '/home/ghoffmn'

• The name means get current working drectory

os.listdir(path)

• os.listdir(path) returns a list of the contents of a directory

  >>> course_dir = os.listdir("/courses/it117/s14/ghoffmn")
  >>> for entry in course_dir :
  ...     print(entry)
  ... 
  GROUP
  MAIL
  cmanuel1
  jpinto
  fortinsy
  ebeazer
  ...

• os.listdir does not return the special entries . and ..
• The list is not in any particular order
• But you can use the built-in function sorted() to change that

os.chdir(path)

• To move to another directory you need to use os.chdir(path)

  >>> os.chdir("/home/ghoffmn/assignments_submitted")
  >>> os.getcwd()
  '/home/ghoffmn/assignments_submitted'

Running Unix Commands within Python

• You can run a Unix command from within Python using os.system()
• The argument to os.system() is a string which contains a Unix command
• os.system() runs the command ...
• and returns the exit status

  >>> result = os.system("touch foo.txt")
  >>> result
  0
  >>> os.listdir(".")
  ['dir1', 'foo.txt']

• When the exit status is 0 the the command ran without error
• If the number is greater than 0 the command did not work
• os.system() will also work with Windows

os.environ

• One of the ways we customize a Unix environment is with shell variables
• os.environ is not a function
• It is a module variable which points to a dictionary
• The keys in the dictionary are names of shell variables
• The values in the dictionary are the values of those variables
• To get the value of a shell variable use the [ ] operator ...
• with the name of the variable inside the square brackets

  >>> os.environ["HOME"]
  '/home/ghoffmn'
  >>> os.environ["SHELL"]
  '/bin/bash'
  >>> os.environ["PATH"]
  '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games'

The os.path Module

• The os.path module contains functions which operate on pathnames
• os.path is part of the os module
• If you import os you will also import os.path

os.path.isfile(path) and os.path.isdir(path)

• os.path.isfile() and os.path.isdir() are boolean functions
• os.path.isfile() returns True if its argument is a file

  >>> os.path.isfile("foo.txt")
  True
  >>> os.path.isfile("dir1")
  False

• os.path.isdir() returns True if its argument is a directory

  >>> os.path.isdir("dir1")
  True
  >>> os.path.isdir("foo.txt")
      False

os.path.basename(path)

• os.path.basename() returns the filename part of a pathname

  >>> os.getcwd()
  '/home/ghoffmn/tmp'
  >>> os.path.basename(os.getcwd())
  'tmp' 

• It should be used in usage messages

The sys Module

• Python scripts run inside two environments
  • The operating system
  • The Python interpreter
• The sys module contains variables and functions ...
• that let you interact with the Python interpreter
• You must import the sys module before you can use it

  >>> import sys

Getting Values from the Command Line

• A script can get the values it needs from the command line
• The sys module contains the variable argv
• sys.argv is a list variable that contains all the command line arguments
• The first command line argument is the pathname used to run the script

Leaving a Running Script

• sys.exit() stops a running script
• You can use it to stop a script before it gets to the end of the code
• Why would you want to do this?
• There are many reasons
• The most common is when your script encounters an error
• Like not getting the command line arguments it needs

The Characters in Regular Expressions

• A regular expression is a string of characters forming a pattern
• This pattern is compared against a string ...
• looking for a match
• If parts of the string that match the regular expression pattern ...
• we have a match
• A regular expression is a string composed of
  • Ordinary characters
  • Meta-characters
  • Character classes

Ordinary Characters in Regular Expressions

• Ordinary characters are characters which are not meta-characters
• An ordinary character will match itself
• So the regular expression "cat" will match the string "cat"
• Regular expressions are case sensitive
• Upper case characters only match upper case characters
• And the same for lower case
• Digits are ordinary characters
• So the regular expression "5" matches the string "5"

Meta-characters in Regular Expressions

• Meta-characters have special meaning in a regular expression
• The meta-characters are

      .  ^  $  *  +  ?  {  }  [  ]  \  |  (  )

• Every character that is not a meta-character is an ordinary character

The . Meta-character

• . matches one of any single character except the newline

The * Meta-character

• * matches zero or more occurrences of the previous character
• * in regular expressions is similar to the * in Unix
• But there is an important difference
• * in Unix matches 0 or more occurrences of any character
• * in regular expressions matches 0 or more occurrences ...
• of the character that comes before it
• To get the same effect as *in Unix you must use .*
• Note that I said zero or more occurrences

The + Meta-character

• The + meta-character is like *
• It is used to indicate repetition of the previous character
• But * means zero or more occurrences
• + means one or more occurrences

The ? Meta-character

• ? is also a repetition meta-character
• It means zero or one occurrences of the previous character
• In other words, it means the previous character is optional

The \ Meta-character

• The backslash, \ , is a meta-character
• It turns off the special meaning of the character that immediately follows it
• It performs the same function as the backslash on the Bash command line
• If you wanted to search for a meta-character
• You would have to put \ in front of it
• The \ is also used in character classes

Character Classes

• Character classes match a single occurence of a set of characters
• A character class is represented by a \ in front of a single letter
• When the letter is lower case the character class matches one occurence ...
• of any character in the set
• When the letter is upper case the character class matches one occurence ...
• of any character not in the lowercase class of the same letter

\d and \D Character Classes

• \d matches a single digit
• \d can be used with a repetition meta-character ...
• to match many occurrences of a digit
• \D matches any single character that is not a digit

The \w and \W Character Classes

• \w matches any single alphanumeric character ...
• and the underscore, _
• The alphanumeric characters are the letters and the digits
• \W matches any single character that is not a letter ...
• or an underscore, _ ...
• or a digit

The \s and \S Character Classes

• \s matches any whitespace character
• \S matches any character that is not whitespace

Getting Strings from a Match

• Regular expressions can be used to get parts of the matching string
• To extract part of a string from a match we need two things
  • The ( ) meta-character
  • The group method of a match object

The ( ) Meta-characters

• We use ( ) to extract part of a matched string
• You place these around the part of the regular expression you want to extract
• We need to do two things
  • Write a regular expression that matches only the line we want
  • Put ( ) meta-characters around the part we want to extract
• Let's say that the the following regular expression matches this line

  \d+\.\d+\.\d+\.\d+.*GET

• To extract the IP address part we would write

  (\d+\.\d+\.\d+\.\d+).*GET

• We can use the group method to get the IP address
• But we have to specify which group we are talking about
• So we have to give group a number
• We only have one set of parentheses
• So we use 1 as the argument to group

  ip_address = match_object.group(1)

Repetition in Regular Expressions

• If we want to match a certain number of digits
• We can use many instances of \d
• Like this

  \d\d\d

• But there is another way
• We can follow \d with curly braces, { }
• And put the number of repetitions we want inside the braces
• Like this

  \d{3}

• This works with all character classes
• And all ordinary characters

Specifying a Range of Repeating Characters

• We can also use curly braces to specify a range of repetitions
• When we do this, the curly braces contain two integers ...
• separated by a comma
• The first integer is the minimum number of repetitions ...
• a nd the second is the maximum
• If we wanted to match either 1, 2, or 3 digits we would write

  \d{1,3}

Creating Custom Character Classes

• Character classes are sets of characters
• Python provides 6 predefined character classes
  • \d matches any digit
  • \D matches any character not a digit
  • \w matches any alphanumeric character and _
  • \W matches any character not an alphanumeric or _
  • \s matches any whitespace character
  • \S matches any character not a whitespace
• But Python lets you define your own character classes
• We do this using the [ ] meta-characters
• The characters you place inside the square brackets
• Are the characters in the character class
• If we wanted match a least one occurence of even digits we would write

  [02468]+

Ranges of Characters in a Character Class

• What if you wanted to match all lowercase letters?
• You can't use \w
• It contains too many other characters
• I can create a custom character class by listing all the lowercase characters

  [abcdefghijklmnopqrstuvwxyz]

• But there is a better way
• I can use a range of characters inside the [ ]
• I do this by placing a - between two characters

  [a-z]

• The characters must be in the correct order

Greedy versus Non-greedy Matching

• There are two repetition meta-characters that can match many characters
  • *
  • +
• By default, any search using these meta-characters will always be "greedy"
• That means that the match will always be as long as possible
• Most of the time this is what you want
• But sometime it isn't
• To do this, put the meta-character ? ...
• after the repetition meta-characters * or +