Week 7

Review, Program Design, and Debugging

The missing line should be:

def double(x):
    """
    Return twice the value of x.
    >>> double(1.2)
    2.4
    >>>
    """
    #...

1. return 2.4
2. return 2 * x
3. return "2 * x"

Answer: B

What's it print?

def f():
    print 'spam'
def g():
    print 'eggs'
h = f
f = g
h()

1. spam
2. eggs
3. nothing, there is an error

Answer: A

What's it print?

def f():
    eggs = 'spam'
def g():
    print eggs
f()
g()

1. spam
2. eggs
3. nothing, there is an error

Answer: C

What's it print?

eggs = 'spam'
def f():
    eggs = 'eggs'
f()
print eggs

1. spam
2. eggs
3. nothing, there is an error

Answer: A

Which of the following statements is a syntax error?

1. pram = 'spam'
2. "spam" = ham
3. "spam = ham"
4. B and C
5. all of the above

Answer: B

Lewis Carroll Quotation

[The White Knight said:] The name of the song is called "Haddocks' Eyes".'

'Oh, that's the name of the song, is it?' Alice said, trying to feel interested.

'No, you don't understand,' the Knight said, looking a little vexed. 'That's what the name is called. The name really is "The Aged Aged Man".'

'Then I ought to have said "That's what the song is called"?' Alice corrected herself.

'No, you oughtn't: that's quite another thing! The song is called "Ways and Means": but that's only what it's called, you know!'

'Well, what is the song, then?' said Alice, who was by this time completely bewildered.

'I was coming to that,' the Knight said. 'The song really is "A-sitting On a Gate": and the tune's my own invention.'

—Through the Looking Glass, chapter 8

Distinctions in the quotation

• What the name of the song is called
• What the name of the song is
• What the song is called
• What the song is

Related variable distinctions

• What a variable's name is: characters in the program parsed as a name
• What a variable is: some contiguous bits in memory
• What a variable's value is: the contents of its memory bits
• What the meaning of a variable name when it appears in a program
  • if the name is on the left-hand-side of an assignment statement: its location in memory
  • if it is a variable reference expression: the contents of its location in memory
  • if it is in a string literal: just the characters in the name

Some reasons why these distinctions matter

• The same variable name can refer to different variable declarations in a program
  • in different scopes
• The meaning of x and "x" in a program are not the same
• There may be multiple references to the same variable
• A local variable reference refers to a different location in memory each time its function is called

Crystal / Mud

On a sheet of paper you can hand in write

• Two or three programming topics that are muddy for you
  • you'd like them reviewed
• Two or three programming topics that are crystal clear for you
  • you don't need them reviewed

Constants

• Variables whose values are not changed after their declaration (first assignment) are "constant"
• However, the term constant usually refers only to variables whose values are given by literals or computations involving other constants
  • their values do not depend on runtime input
  • example: dollars_per_euro in Assignment 4
• Constants should be global, since there is no point in creating variables for them every time a function is called
• It is customary style to define a program's constants in a series of declarations grouped right after the program imports at the start of a program

Program element order

It is good style to order elements of your program file as follows (not all elements may be present in a program, and additional comments may be placed anywhere they are a help):

1. Initial comment as described in point 1 above
2. Import statements
3. Constant declarations
4. Global variable declarations
5. Function definitions
   • constants, imports, and variables that are only used by one function may be placed just before the function
6. Statements that execute the program
   • often just a single call to a function named main

Any order is possible, as long as variables are assigned before they are referenced

• recall that function bodies are not executed when the function is defined

Good program development strategy

• Top-down design
  • write pseudo code that captures the whole program
  • break it into logically related pieces (divide and conquer strategy)
  • rewrite the pieces in forms that are closer and closer to Python code (stepwise refinement strategy)
• Valuable variation: incremental testing
  • simplify some pieces drastically to allow testing of other parts early in the program's development
    • have at least a general idea what will be required of the simplified piece in the end
• Possible problem: it may turn out that you cannot implement some piece that is needed (due to lack of skill, practical limitations, or theoretical limitations)

Another program development strategy

• Bottom-up design
  • write and test pieces that you believe will be be useful in the final program
  • combine pieces to form larger pieces until the program is complete
• Danger: you may spend a lot of time doing something that turns out not to be needed, or the details of which do not fit the problem at hand

Combining top-down and bottom-up design

• Use bottom-up design for pieces that use technology you are not sure you understand or algorithms you are not sure you can implement
  • because of practical or theoretical constraints, or because you lack the training and experience
• Use top-down design for the rest (hopefully for most of the program)

One way not to develop programs

Sequential coding: write lots of code without a plan and without testing it until you hope you are done

• you may discover you are on the wrong track and have to throw out a lot of code
• debugging is very difficult because errors may be anywhere

Another way not to develop programs

Trial and error programming: try different thinks you think might work until something does work

• may work for very small programs that can be solved using a very small number of language features
  • may even work when you do not understand key language features!
• quickly becomes extremely time consuming and then impossible as program size and the number of language features increases beyond very elementary levels
  • The number of possibilities grows exponentially with both the number of language features and the program size!
• does not force you to learn key language features
• necessary on occasion when you encounter undocumented system behavior
  • not the case in an introductory course

Which of the following is a legal string?

1. 'Wha'd you say?'
2. 'She said "What the @!#$!".'
3. "He said "That's not good."."
4. "Here we\ygo again!"

Answer: B

What does the following program print?

s = 'a'
s = s + s
s = s + 'b'
print s + 'c'

1. 'aaabc'
2. 'aabc'
3. 'abc'
4. 'ac'
5. nothing: there is an error

Answer: B

Which of the following can be in a Python expression?

1. or
2. if
3. return
4. while
5. A and B
6. A, B and C
7. all of the above
8. none of the above

Answer: A

The value of 4/2 == 3 or 2 or 1 is

1. a true value
2. a false value
3. niether a true or false value
4. there is a syntax error

Answer: A

What does it print?

x = 3
def f(x):
    x = 4
f(x)
print x

1. 3
2. 4
3. nothing: there is a syntax error
4. nothing: there is a runtime error

Answer: A

Another "what does it print?"

x = 3
def f():
    x = 4
f()
print x

1. 3
2. 4
3. nothing: there is a syntax error
4. nothing: there is a runtime error

Answer: A

Yet another "what does it print?"

i = 0
s = ''
while i < 5:
    if i % 2:
        s = s + str(i) + ' '
    i = i + 1
print s

1. 1
2. 1 2
3. 1 3
4. 1 2 3
5. 1 3 5

Answer: C

Have you tried to use the Python Syntax course web page?

1. yes, and it is helpful
2. yes, I understand it but I don't find it helpful
3. yes, but I don't understand how to use it
4. no, but I've been meaning to
5. no, and I don't want to

Functional composition example

The following program has a function harmonic_mean that computes the mathematical harmonic mean of two numbers

• the harmonic_mean function calls the function inverse (twice): functional composition
• the program ends with a couple of test calls

# harmonic_mean.py, by chaynes@indiana.edu

def harmonic_mean(x, y):
    return 2 / (inverse(x) + inverse(y))

def inverse(x):
    return 1 / x

print harmonic_mean(3.0, 2.0)

print harmonic_mean(3, 2)

• running this program results in the following output:

  2.4
  
  Traceback (most recent call last):
    File "C:\home\201\c\6\harmonic_mean.py", line 11, in -toplevel-
      print harmonic_mean(3, 2)
    File "C:\home\201\c\6\harmonic_mean.py", line 4, in harmonic_mean
      return 2 / (inverse(x) + inverse(y))
  ZeroDivisionError: integer division or modulo by zero
  

Function composition

• One function calling another is called function composition.
  • it is very common and there is no limit to how many times this can happen before one of the functions returns
• The system maintains a call stack that records, for each call that has not yet returned, where the call was made and the the values of variables accessible at the point of call
• When a runtime error occurs, a Traceback is printed that lists each of the calls on the call stack (oldest first)
  • for each call the line of code that made the call is printed with its line number in the code file
  • with the IDLE debugger it is possible to inspect the values of the variables at each point of call (we'll see that in action later)

Example: countdown

def countdown(n):
    """
    Print a line with the integers n, n-1, ..., 1, separated by spaces.
    >>> countdown(5)
    >>>
    """
    s = ''
    while n >= 1:
        s = s + str(n)
        if n != 1:
            s = s + ' '
        n = n - 1
    print s

Python input/output

• Every user program in a computer, such as your Python programs, are managed by the operating system as a process
• Program interactions with the world outside of its own process are broadly referred to as input/output, or I/O
• All I/O is managed by the operating system
• Python programs can potentially perform any kind of I/O supported by the operating system
• Kinds of I/O used in this course
  • character (plain text) I/O to and from an operating system or Python programming environment "shell"
    • raw_input function for input
    • print statement for output
  • file I/O (reading and writing plain text files)
  • Internet I/O (reading a web page)
  • displaying an image using the Python Image Library (if we have time)