Learn Python the Hard Way

Exercises 20 – 26

http://learnpythonthehardway.org/

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()

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

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

Examples using f.readline()

What is the result?

a) case = 5case = case + 1

b) case = 5case += 1

c) case = 5case –= 1

What is the result?

a) case = 5case = case + 1

b) case = 5case += 1

c) case = 5case –= 1

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”

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

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.

Exercise 21

See if you can figure this out.

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?

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))))

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.

Exercise 21

Exercise 22: Do it or don’t do it.

Exercise 22

Exercise 22: Do it or don’t do it.

Exercise 22

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.

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

Exercise 23

http://www.codeschool.com/courses/try-git

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: """

Exercise 24

Exercise 24

Exercise 24

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

Exercise 24

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

Exercise 24

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

A little break: TextWrangler settings

Setting preferences in TextWrangler:

1. Line numbers (turn them on)2. Soft wrap (lines)3. Appearance of soft-wrapped

lines4. Colors

TextWrangler – turn on the line numbers

TextWrangler – “soft wrap” lines

Note: You can also change the font size here. Make it larger if you are feeling eye strain.

TextWrangler – appearance of “soft wrap”

TextWrangler – change the colors

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

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

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

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

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?

Exercise 25

What does this teach you about the way the function sort_words() really works?

Exercise 25

Do you understand it now?

Exercise 25

Ex. 25: What does each one do?

(something is a variable name)

something.split(' ')sorted(something)something.pop(0)something.pop(–1)

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.

Exercise 25

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.

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

Exercise 26

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

Exercise 26

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

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]

Learn Python the Hard Way

Exercises 20 – 26

(we are getting smarter, little by little)