Based on Zed Shaw's "Learn Python the Hard Way," this is a review of Exercises 20 - 26 in that text. For non-computer-science students and learners.

1. Learn Python the Hard Way
Exercises 20 – 26
http://learnpythonthehardway.org/

2. Review: Things we do with files
• Open: f = open(somefile)
• Read: f.read()
• Erase: f.truncate()
• Write: f.write()
• Close: f.close()
• Rewind: f.seek(0)
• Move the “playhead”: f.seek()
• Read the current line: f.readline()

3. Note that
f.seek(0)
returns to
the start of
the file …

4. … but with
another
number, such as
f.seek(52)
… reading the
file will begin at
that position,
not at the start.

5. Examples using
f.readline()

6. What is the result?
a) case = 5
case = case + 1
b) case = 5
case += 1
c) case = 5
case –= 1

7. What is the result?
a) case = 5
case = case + 1
b) case = 5
case += 1
c) case = 5
case –= 1

8. Using pydoc
• Zed suggests you look at the built-in Python
documentation from time to time.
• When he says “try pydoc file” — do this:
– Make sure you are not in Python (look for the $)
– Type:
python –m pydoc file
– This will give you the help document about “file”

9. Python’s built-in documentation:
Press the down arrow to scroll.
Press Q to quit.

10. Exercise 21
Returns
• When you want to run a function and use the
result outside the function, you’ll put a
return statement inside the function.
• When the function reaches the line with the
return statement, the function will stop
running (and return the specified value).
• When you call a function that includes a
return statement, you must have a variable
name to “catch” the returned value.

11. See if you can figure this out.

12. Sometimes exercises for beginners are a bit weird.
You wouldn’t write a program like this to do real work.
But it’s a good example for trying to wrap your brain around the
idea of returns.
NOTICE: Are x and y used outside the function?

14. Exercise 21
Exercise 21’s study drill is tough,
especially if you are
easily confused by math.
what = add(age, subtract(height, multiply(weight, divide(iq, 2))))

15. Exercise 21
what = add(age, subtract(height, multiply(weight, divide(iq, 2))))
There’s no point in anyone explaining it.
Just try your best to follow
Zed’s instructions.
Like he says, it’s a puzzle.

17. Exercise 22
Exercise 22: Do it or don’t do it.

18. Exercise 22
Exercise 22: Do it or don’t do it.
I did it. Just what Zed says. You should too.
If you haven’t done it yet, then put it on your
To Do list for this week.

20. Exercise 23
Exercise 23: Reading code.
(Other people’s code.)

24. Exercise 24
Practicing what you (should) know
• Review of how escape characters work:
n newline
t tab indent
one backslash
' one single quote
• How the “triple double-quotes” work: """

25. Exercise 24

26. Exercise 24
This is close to Zed’s version. Do not be fooled by his re-use of variable names.

27. Exercise 24
Note how in this version, I changed the variable names. And it works the same way.

28. Exercise 24
This version is the most confusing, but it’s also the most concise.

29. A little break: TextWrangler settings
Setting preferences in TextWrangler:
1. Line numbers (turn them on)
2. Soft wrap (lines)
3. Appearance of soft-wrapped
lines
4. Colors

30. TextWrangler – turn on the line numbers

31. TextWrangler – “soft wrap” lines
Note: You can also change the
font size here. Make it larger
if you are feeling eye strain.

32. TextWrangler – appearance of “soft wrap”

33. TextWrangler – change the colors

34. Ex. 25: You made a “module”!
• You wrote a bunch of functions in one .py file
• But … the file does not call any of the
functions
• Thus, no functions run when you run the file

35. Ex. 25: You made a “module”!
• You wrote a bunch of functions in one .py file
• But … the file does not call any of the
functions
• Thus, no functions run when you run the file
• Remember from sys import argv ?
• Now you are importing some things you wrote
yourself

36. This is what I get when I run: help(ex25)

37. That is because I wrote THIS in my ex25.py file!

38. Exercise 25
This is slightly different from what Zed suggests,
because I learn a lot from playing with the code.
Look at the line that gave me an error.
Why did I get that error?
How did I fix it?

39. Exercise 25
What does this teach you about the way the function
sort_words() really works?

40. Exercise 25
Do you understand it now?

41. Ex. 25: What does each one do?
(something is a variable name)
something.split(' ')
sorted(something)
something.pop(0)
something.pop(–1)

42. Exercise 25
This is how I answer the questions on the previous slide.
When Zed says “this is a list which you will learn about
later,” he means the result at the arrow, above.

43. Ex. 26: Fix someone else’s code
• You might not enjoy this exercise, but (like
medicine) it is good for you
• Remember: Zed is showing you how to learn
• This is why I chose Zed’s book
P.S. When I did this exercise, it took me 17
minutes altogether. But I spent a long, long time
on exercises 24 and 25.

44. Exercise 26
(1) This is how the program ran after I had fixed all the errors.

45. Exercise 26
(2) This is how the program ran after I had fixed all the errors.

46. Heads up! Exercise 27 is important. Zed asks you to memorize and tells you how.
Do what he says.

47. Some students have paid $29 to download Zed’s
videos. You also get a PDF of the complete book,
Learn Python the Hard Way. It’s a complete
package, all videos and the PDF for one price.
[LINK]

48. Learn Python the Hard Way
Exercises 20 – 26
(we are getting smarter, little by little)