Exercise 25: Even More Practice
We're going to do some more practice involving functions and variables to make sure you know them well. This exercise should be straightforward for you to type in, break down, and understand.
However, this exercise is a little different. You won't be running it. Instead you will import it into Python and run the functions yourself.
First, run this like normal with python ex25.py to find any errors you have made. Once you have found all of the errors you can and fixed them, you will then want to follow the WYSS section to complete the exercise.
What You Should See
In this exercise we're going to interact with your ex25.py file inside the python interpreter you used periodically to do calculations. You run python from the terminal like this:
$ python Python 2.7.1 (r271:86832, Jun 16 2011, 16:59:05) [GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>>
Yours should look like mine and after the > character (called the prompt) you can type Ruby code in and it will run immediately. Using this I want you to type each of these lines of Ruby code into python and see what it does:
Here's what it looks like when I do it with the ex25.py module:
Python 2.7.1 (r271:86832, Jun 16 2011, 16:59:05) [GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> import ex25 >>> sentence = "All good things come to those who wait." >>> words = ex25.break_words(sentence) >>> words ['All', 'good', 'things', 'come', 'to', 'those', 'who', 'wait.'] >>> sorted_words = ex25.sort_words(words) >>> sorted_words ['All', 'come', 'good', 'things', 'those', 'to', 'wait.', 'who'] >>> ex25.print_first_word(words) All >>> ex25.print_last_word(words) wait. >>> words ['good', 'things', 'come', 'to', 'those', 'who'] >>> ex25.print_first_word(sorted_words) All >>> ex25.print_last_word(sorted_words) who >>> sorted_words ['come', 'good', 'things', 'those', 'to', 'wait.'] >>> sorted_words = ex25.sort_sentence(sentence) >>> sorted_words ['All', 'come', 'good', 'things', 'those', 'to', 'wait.', 'who'] >>> ex25.print_first_and_last(sentence) All wait. >>> ex25.print_first_and_last_sorted(sentence) All who
As you go through each of these lines, make sure you can find the function that's being run in ex25.py and you understand how each one works. If you get different results or error, you'll have to go fix your code, exit python and start over.
- Take the remaining lines of the WYSS output and figure out what they are doing. Make sure you understand how you are running your functions in the ex25 module.
- Try doing this: help(ex25) and also help(ex25.break_words). Notice how you get help for your module, and how the help is those odd """ strings you put after each function in ex25? Those special strings are called documentation comments and we'll be seeing more of them.
- Typing ex25. is annoying. A shortcut is to do your import like this: from ex25 import * which is like saying, "Import everything from ex25." Programmers like saying things backward. Start a new session and see how all your functions are right there.
- Try breaking your file and see what it looks like in Python when you use it. You will have to quit Python with CTRL-D (CTRL-Z on windows) to be able to reload it.
Common Student Questions
- I get a None printed out for some of the functions.
- You probalby have a function that is missing the return at the end. Go backward through the file like I taught you and confirm that every line is right.
- I get -bash: import: command not found when I type import ex25.
- Pay attention to what I'm doing in the WYSS section. I'm doing this in Python not in the Terminal. That means you first run Python.
- I get ImportError: No module named ex25.py when I type import ex25.py.
- Don't add the .py to the end. Python knows the file ends in .py so you just type import ex25.
- I get SyntaxError: invalid syntax when I run this.
- That means you have something like a missing ( or " or similar syntax error on that line or above it. Any time you get that error, start at the line it mentions and check that it's right, then go backward checking each line above that.
- How can the words.pop(0) be changing the words variable then?
- That's a complicated question, but in this case words is a list, and because of that you can give it commands and it'll retain the results of those commands. This is similar to how files and many other things worked when you were working with them.
- When should I print vs. return in a function?
- You need to understand that print is only for printing to the screen and that you can actually do both print and return a value. When you understand this then you'll see that the question is kind of a pointless question. You use print when you want to print. You use return when you want to return.