CHAPTER 4. THE PYTHON INTERPRETER 17
Listing 4.1: Example Python interpreter
Python 2 . 5 . 1 ( r251 : 5 4 8 63 , Jan 17 2008 , 1 9 : 3 5 : 1 7 )
[GCC 4 . 0 . 1 ( Apple Inc . b u i l d 5 4 6 5) ] on darwin
Type " h elp " , " c o pyri g h t " , " c r e d i t s " or " l i c e n s e " for more i nfor m a t i on .
>>>
You may notice that the Python version used in the above examples is 2.5.1; the current version is 3.0.
Don’t panic. The vast majority of what you will learn will work regardless of what version you’re using. My
favorite Python book, Python How to Program, is based on version 2.2 but I still use it nearly every day as
a reference while coding.
For the most part, you won’t even notice what version is in use, unless you are using a library, function,
or method for a specific version. Then, you simply add a checker to your code to identify what version the
user has and notify him to upgrade or modify your code so it is backwards-compatible.
(A later chapter in this book will cover all the major differences you need to be aware of when using
Python 3. Right now you’re just learning the basics that you will use regardless of which version of the
language you end up using.)
The >>> in Listing 4.1 is the Python command prompt; your code is typed here and the result is
printed on the following line, without a prompt. For example, Listing 4.2 shows a simple print statement
from Python 2.5:
Listing 4.2: Python command prompt
>>>print "We a re the kni g h ts who say , ’ Ni ’ . "
We are the k n i ghts who say , ’ Ni ’ .
4.2 Python Versions
As an aside, Python 3 has changed print from a simple statement, like Listing 4.2, into a bona-fide function
(Listing 4.3). Since we haven’t talked about functions yet, all you need to know is that Python 3 simply
requires you to use the print statement in a slightly different way: parenthesis need to surround the quoted
words. Just so you’re aware.
Listing 4.3: Print function (Python 3)
>>>print ( "We a re the kni g h t s who say , ’ Ni ’ . " )
We are the k n i ghts who say , ’ Ni ’ .
The vast majority of the code in this book will be written for Python 2.6 or earlier, since those versions
are installed by default on many *nix systems and is therefore still very popular; there is also a lot of older
code in the wild, especially open-source programs, that haven’t been (and probably never will be) upgraded
to Python 3. If you decide to use other Python programs to study from or use, you will need to know the
“old school” way of Python programming; many open-source programs are still written for Python 2.4.
If you are using Python 3 and you want to type every example into your computer as we go along, please
be aware that the print statements, as written, won’t work. They will have to be modified to use a print()
function like in Listing 4.3.
4.3 Using the Python Command Prompt
If you write a statement that doesn’t require any “processing” by Python, it will simply return you to the
prompt, awaiting your next order. The next code example shows the user assigning the value “spam” to the
variable can. Python doesn’t have to do anything with this, in regards to calculations or anything, so it
accepts the statement and then waits for a new statement.
Listing 4.4: Python statements
>>>can = "spam"
>>>
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