Before we learn about the actual Python language, we need to know where to enter it. Python can be used either interactively, or from scripts. There is not one right way to do it, and it depends whether you are interested in an interactive exploratory analysis, or running or re-running a more complex program.
To run Python code interactively, one can use the standard Python prompt, which can be launched by typing python in your standard shell:
$ python
Python 2.7.2 (default, Nov 5 2011, 20:09:20)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
The >>> indicates that Python is ready to accept commands. If you type a=1 then press enter, this will assign the value 1 to a. If you then type a you will see the value of a (this is equivalent to print a):
>>> a = 1
>>> a
1
The Python shell can execute any Python code, even multi-line statements, though it is often more convenient to use Python non-interactively for such cases.
The default Python shell is limited, and in practice, you will want instead to use the IPython (or interactive Python) shell. This is an add-on package that adds many features to the default Python shell, including the ability to edit and navigate the history of previous commands, as well as the ability to tab-complete variable and function names. To start up IPython, type:
$ ipython
Python 2.7.2 (default, Nov 5 2011, 20:09:20)
Type "copyright", "credits" or "license" for more information.
IPython 0.11 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
In [1]:
The >>> symbols are now replaced by In [x], and output, when present, is prepended with Out [x]. If we now type the same commands as before, we get:
In [1]: a = 1
In [2]: a
Out[2]: 1
If you now type the up arrow twice, you will get back to a = 1.
Note
IPython is much more than just a Python shell with a few improvements compared to the default shell. See the IPython documentation for a full overview of its capabilities!
While the interactive Python mode is very useful for exploring and trying out code, you will eventually want to write a script to record and reproduce what you did, or to do things that are too complex to type in interactively (defining functions, classes, etc.). To write a Python script, just use your favorite code editor to put the code in a file with a .py extension. For example, we can create a file called test.py containing:
a = 1
print a
We can then run the script on the command-line with:
$ python test.py
1
Note
The print statement is necessary, because typing a on its own will only print out the value in interactive mode. In scripts, the printing has to be explicitly requested with the print command. To print multiple variables, just separate them with a comma after the print command: print a, 1.5, "spam". To print variables within strings use the following syntax: print ("This is a integer: %d, this is a float: %f, and is a string: %s" % (5, 3.141, "spam"))
It is also possible to make Python scripts executable. Simply add #!/usr/bin/env python on the first line of your test.py script and change the file permission to make it executable with chmod +x test.py. Now the script can be run without the preceeding python command; instead you can just type ./test.py in the command line. Note that this will only work on Linux and Macs, not on Windows.
It can sometimes be useful to run a script to set things up, and to continue in interactive mode. This can be done using the %run IPython command to run the script, which then gets executed. The IPython session then has access to the last state of the variables from the script:
$ ipython
Python 2.7.2 (default, Nov 5 2011, 20:09:20)
Type "copyright", "credits" or "license" for more information.
IPython 0.11 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
In [1]: %run test.py
1
In [2]: a + 1
Out[2]: 2