Running Python
To start using Python, we need access to the python program in a terminal. If you have a reasonable recent linux flavor at your disposal, the python version that you are running should be new enough to follow these lessons. The same goes for the python version version that ships with aliBuild.
Python 3?
You might see material that talks about Python 3. Like a lot of other software, Python is regularly updated and groups batches of updates, including bug fixes and new features, into versions. The interesting thing about Python 3 is that it isn’t backwards compatible with Python 2. This means that code that works when run with version 2 of Python may not necessarily work when run with version 3. Python 2 was around for a long time, and so the process of migrating to Python 3 has been slow, which is why so many people talk about it.
We use Python 2 in these lessons. In general, and out in the real world, Python 3 is preferred as it receives the most focus from the people who make Python. If you’re starting a new project, and don’t have to use LHCb software, consider trying to use Python 3. You can install both Python 2 and Python 3 using Anaconda.
Python is a very user-friendly language. If you’re used to having to compile your code, this might seem refreshing:
$ python
Python 2.7.13 (default, Dec 5 2017, 19:29:24)
[GCC 6.2.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 3.14
>>> print a + 1
4.14
>>> a
3.14
Woah! What just happened?
- We started an interactive Python session, also known as a Python shell,
by executing the
pythoncommand; - We typed a line of code,
a = 3.14, and hit enter; - We typed another line of code,
print a, and hit enter; - The value
3.14was printed to the terminal.
This interactive session is sometimes called a REPL: a Read Evaluate
Print Loop. This is just like bash, where you type your command, run
it, see the results, and can then type the next line. Sometimes there are no
results, so you don’t see anything being printed (just like running the true
command in bash).
You can leave the session by running exit(), or by using the Ctrl-d key
combination.
Everything that can be done in Python can be done in an interactive session; it’s a great way to experiment. An enhanced version of this session is called IPython.
$ ipython
Python 2.7.13 (default, Dec 5 2017, 19:29:24)
Type "copyright", "credits" or "license" for more information.
IPython 5.4.1 -- 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]: print 1 + 3
4
There are a few advantages to using ipython over python:
- Command history persists across sessions. This also works just like
bash: hit the up arrow to go through lines you typed in the past. If you already have part of a command typed out, and then hit the up arrow, IPython will only show you lines that started with the same characters. - Autocompletion. If you hit the
tabkey whilst typing something, IPython will present you with a list of things that match the word you’re in the middle of typing.
In [2]: abc_my_var = 3.14
In [3]: abc_<tab>
In [4]: import math
In [5]: math.s<tab>
math.sin
math.sinh
math.sqrt
- Easily run shell commands by starting your line with an exclamation mark.
In [6]: !cal
October 2017
Su Mo Tu We Th Fr Sa
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31
- Access the value of the last line with the special
_variable
In [7]: 3.14 + 4.13
7.27
In [8]: _
7.27
Your best friend in a (I)Python shell is the help method. If you want more
information on something, just ask for help!
In [7]: help()
In [8]: help(math)
You can see what names are available to you by using the dir method.
In [9]: dir()
In [10]: __doc__
'Automatically created module for IPython interactive environment'
In [11]: dir(math)
['__doc__',
...
'sin',
...
'trunc']