Rules For If-Statements
1.Every if-statement must have an else.
2.If this else should never be run because it doesn't make sense, then you must use a die function in the else that prints out an error message and dies, just like we did in the last exercise. This will find many errors.
3.Never nest if-statements more than 2 deep and always try to do them 1 deep. This means if you put an if in an if then you should be looking to move that second if into another function.
4.Treat if-statements like paragraphs, where each if,elif,else grouping is like a set of sentences. Put blank lines before and after.
5.Your boolean tests should be simple. If they are complex, move their calculations to variables earlier in your function and use a good name for the variable.
Rules For Loops
1.Use a while-loo... (查看原文)
From now on, when you make mistakes write down on a piece of paper what kind of mistake you made.
When you go to the next exercise, look at the last mistakes you made and try not to make them in this new one. (查看原文)
The argv is the "argument variable", a very standard name in programming, that you will find used in many other languages. This variable holds the arguments you pass to your Python script when you run it. In the exercises you will get to play with this more and see what happens.
Line 3 "unpacks" argv so that, rather than holding all the arguments, it gets assigned to four variables you can work with: script, first, second, and third. This may look strange, but "unpack" is probably the best word to describe what it does. It just says, "Take whatever is in argv, unpack it, and assign it to all of these variables on the left in order." (查看原文)
You should immediately notice that we import another handy command named exists. This returns True if a file exists, based on its name in a string as an argument. It returns False if not.
Did you see that trick I did with cat? It only works on Linux or OSX, on Windows use type to do the same thing. (查看原文)
Functions do three things:
1.They name pieces of code the way variables name strings and numbers.
2.They take arguments the way your scripts take argv.
3.Using #1 and #2 they let you make your own "mini scripts" or "tiny commands".
You can create a function by using the word def in Python. (查看原文)
This shows all different ways we're able to give our function cheese_and_crackers the values it needs to print them. We can give it straight numbers. We can give it variables. We can give it math. We can even combine math and variables. (查看原文)
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. (查看原文)
A colon at the end of a line is how you tell Python you are going to create a new "block" of code, and then indenting 4 spaces tells Python what lines of code are in that block. (查看原文)
To avoid these problems, there's some rules to follow:
1.Make sure that you use while-loops sparingly. Usually a for-loop is better.
2.Review your while statements and make sure that the thing you are testing will become False at some point.
3.When in doubt, print out your test variable at the top and bottom of the while-loop to see what it's doing.
If at any time that you are doing this it goes crazy (it probably will), just hold down CTRL and hit c (CTRL-c) and the program will abort. (查看原文)
When you write comments, describe why you are doing what you are doing. The code already says how, but why you did things the way you did is more important. (查看原文)
If you are reading this book and flipping out at every third sentence because you feel I'm insulting your intelligence, then I have three points of advice for you:
- Stop reading my book. I didn't write it for you. I wrote it for people who don't already know everything.
- Empty before you fill. You will have a hard time learning from someone with more knowledge if you already know everything.
- Go learn Lisp. I hear people who know everything really like Lisp.
For everyone else who's here to learn, just read everything as if I'm smiling and I have a mischievous little twinkle in my eye. (查看原文)
This book's job is to teach you the three most essential skills that a beginning programmer needs to know: reading and writing, attention to detail, and spotting differences. (查看原文)
Study Drills
Find out if you were right about what the # character does and make sure you know what it's called (octothorpe or pound character).
Take your ex2.py file and review each line going backward. Start at the last line, and check each word in reverse against what you should have typed.
Did you find more mistakes? Fix them.
Read what you typed above out loud, including saying each character by its name. Did you find more mistakes? Fix them.
Why do I have to read code backward?
It's a trick to make your brain not attach meaning to each part of the code, and doing that makes you process each piece exactly. This catches errors and is a handy error-checking technique. (查看原文)
Keywords
KEYWORD DESCRIPTION EXAMPLE
and Logical and. True and False == False
as Part of the with-as statement. with X as Y: pass
assert Assert (ensure) that something is true. assert False, "Error!"
break Stop this loop right now. while True: break
class Define a class. class Person(object)
continue Don't process more of the loop, do it again. while True: continue
def Define a function. def X(): pass
del Delete from dictionary. del X[Y]
elif Else if condition. if: X; elif: Y; else: J
else Else condition. if: X; elif: Y; else: J
except If an exception happens, do this. except ValueError, e: print e
exec Run a string as Python. exec 'print "hello"'
finally Exceptions or not, finally do this no matter what. finally: pass
for Loop over a collection of things. for X in Y: pass
from Importing ... (查看原文)
