The reading for this assignment is Chapters 1 and 2 of
How to think.... The goal of this assignment is to
explore the Python interpreter, to make some syntax and
run-time errors, and to write and execute a Python script.
- To start Python, type python in a Unix shell. You should get a
startup message and a chevron (»>):
Python 2.3.3 (#1, May 7 2004, 10:31:40)
[GCC 3.3.3 20040412 (Red Hat Linux 3.3.3-7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
The chevron is a prompt; it means that the interpreter is waiting
for you to type an expression or a statement.
- Type 1 + 2 and then hit return. Python evaluates this
expression, prints the result, and then prints another prompt.
- Type 1 2 and then hit return. Python tries to evaluate
the expression, but it can't because the expression is not syntactically
legal. Instead, it prints the error message:
File "<stdin>", line 1
SyntaxError: invalid syntax
In many cases, Python indicates where the syntax error occurred, but
it is not always right, and it doesn't give you much information about
what is wrong. So, for the most part, the burden
is on you to learn the syntax rules.
In this case, Python is complaining because there is no operator between
the numbers. If you want to perform multiplication, you have to
use the multiplication operator, which is *.
- Type print 'hello'. Python executes this statement, which
has the effect of printing the letters hello. Notice that the
quotation marks that you used to enclose the string are not part of
- Type print bob without the quotation marks. The output
will look something like this:
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'bob' is not defined
This is a run-time error; specifically, it is a NameError, and even
more specifically, it is an error because the name bob is not
defined. If you don't know what that means yet, you will soon.
When you are only evaluating a few expressions, or executing a few
statements, it is convenient to use the Python interpreter in
interactive mode. When you start writing bigger programs, you
will want to store your programs in a file called a script
and then use the interpreter to execute the script.
- An easy way to create and edit files is with
emacs. When I am working on a program, I usually have an
emacs window to edit the program and a terminal window
to run it.
Make sure that you are in the directory where you want to put a new
file and type emacs test.py &. Because the file name ends with
py, emacs knows that it is a Python program. The
ampersand is there so you can use the emacs window and the terminal at
the same time.
- Write a line of Python code in the emacs window, maybe a
print statement like print 'hello'
- Save the file using the File menu, the disc icon or
the keyboard shortcut Control-x Control-s.
- In the terminal window, type python test.py. You are
starting the interpreter and telling it to execute the script
test.py. It should print hello.
- In the emacs window, change hello to jello.
Now go back to the terminal window and run the script again. Did
it say hello or jello? If you saved the script before
you ran it again, you will see the new version; otherwise you
will see the old version.
This is, frankly, an annoying little GOTCHA! You have to remember
to save your script before you execute it.
- Add a second line to your script, something like print 3+1.
Save the file and then go back to the terminal. Press the Up Arrow
key. You should see the previous command again, and you can hit return
to execute it again. That should save some typing.
- We're almost done, but there is one more GOTCHA! I have to
warn you about. Add another line to your script, but instead of
a statement (like a print statement), just write an expression
(like 5*7). Save and run the script. Did it print 35?
Probably not. The reason is that when you evaluate an expression in a
script, Python doesn't display the result. It you want to
display the result, you have to use a print statement.
- At this point, you can create and execute Python scripts, and
that's pretty much all you need to know for the rest of the
semester. Of course, there are other things you might want
- Make sure you are in a directory where you want to put
Python code and then use wget to download World.py
from the class web page:
Now press the Up Arrow key and edit the previous command so
that it reads
Press return to download AmoebaWorld.py.
- Run the program you just downloaded by typing
You should see the top view of a microscope slide with an amoeba in
- Press the Run button. The amoeba should move toward
the northeast corner of the slide, leaving a trail of slime.
- Change the entry marked x(t) so that it reads
cos(t) and change y(t) to sin(t). Press run again.
In the next homework we will do something more serious with
AmoebaWorld but for now I thought you would like to see it.