Learning Python - Week 3

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.

What is the result?
a) case = 5
case = case + 1

b) case = 5
case += 1

c) case = 5
case –= 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.

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.

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

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 22

Exercise 22: Do it or don’t do it.

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.

Exercise 23

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

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: """

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
lines
4. 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!

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?

Exercise 25

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

Exercise 25

Do you understand it now?

Ex. 25: What does each one do?
(something is a variable name)

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

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.

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.

Exercise 26

(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.

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)

Learning Python - Week 3

Recommandé

Recommandé

Contenu connexe

Plus de Mindy McAdams

Plus de Mindy McAdams (20)

Dernier

Dernier (20)

Learning Python - Week 3

Notes de l'éditeur