Chapter 1 exercise set 0: Chapter Review¶
Using the Python interpreter, type
1 + 2 and then hit return. Python
evaluates this expression, displays the result, and then shows another
* is the multiplication operator, and
** is the
exponentiation operator. Experiment by entering different expressions and
recording what is displayed by the Python interpreter.
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 shows
the error message:
File "<interactive input>", line 1 1 2 ^ 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.
See if you can find a few more examples of things that will produce error messages when you enter them at the Python prompt. Write down what you enter at the prompt and the last line of the error message that Python reports back to you.
print("hello") at the Python prompt. Python executes this, which has
the effect of printing the letters h-e-l-l-o. Notice that the quotation marks
that you used to enclose the string are not part of the output. Now type
"hello" and describe your result. Make notes of when you see the quotation
marks and when you don’t.
cheese without the quotation marks. The output will look something
Traceback (most recent call last): File "<interactive input>", line 1, in ? NameError: name 'cheese' 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 cheese is not defined. If you don’t know what that means yet, you will soon.
Shell vs. script mode¶
6 + 4 * 9 at the Python prompt and hit enter. Record what happens.
Now create a python script named
test1.py with the following contents:
6 + 4 * 9
What happens when you run this script? Now change the contents to:
print(6 + 4 * 9)
and run it again. What happened this time?
Whenever an expression is typed at the Python prompt, it is evaluated and the result is automatically shown on the line below. (Like on your calculator, if you type this expression you’ll get the result 42.)
A script is different, however. Evaluations of expressions are not automatically displayed, so it is necessary to use the print function to make the answer show up.
It is hardly ever necessary to use the print function in shell mode.
Getting to know
If you are using a GNU/Linux system, especially if it is Ubuntu, instructions for setting up your system are in Appendix A: Configuring Ubuntu for Python Web Development. Now would be a great time to read through Vim and $HOME environment.
If you are on another kind of system, you will need a text editor with which you are comfortable to write your Python scripts.
Write a program named
using_print1.py that displays the following then
run from the command prompt (with
This is line 1. This is line 2, with a blank line above it.