python
single: IPython; IPython Notebook
IPython notebooks are useful for logging your work. Here we suggest that you use them for logging and turning in homework assignments. You may also find them useful in other contexts such as laboratory work. The IPython Notebook interface is similar to Mathematica or Maple.
An IPython notebook is a web-based environment for interactive computing. You can work in this environment interactively just as you would using the IPython shell. In addition, you can also store and run programs in an IPython notebook just like you would from the Spyder editor window. Thus, it would seem that an IPython notebook and the Spyder do essentially the same thing. Up to a point, that is true. In the final analysis, however, Spyder (or some similar Python development environment) is generally more useful for developing, storing, and running code. An IPython Notebook, on the other hand, is excellent for logging your work in Python. That is why we suggest using them for homework assignments. You may find them useful in other contexts as well.
If you have installed Anaconda, then you have already installed all the
software you need to use IPython notebooks. IPython notbooks are stored
in files with a .ipynb extension. To create an IPython notebook,
launch the Terminal (Mac) or the Command Prompt (PC) application. On the
Mac, the Terminal application is found in the Applications/Utilies
folder. On the PC, the Command Prompt application is found under the
Start/All Programs/Accessories menu. Here we will refer the the Terminal
or Command Prompt applications at the System Console. Type pwd (Mac)
or chdir (PC) to determine the current directory (folder) of the
System Console. Type ls (Mac) or dir (PC) to list all the files and
subdirectories in the current directory. Using the cd change directory
command, move the System Console to the directory in which you want to
store your IPython notebook.
At the System Console prompt, type
ipython notebook --matplotlib inline &
This will launch the IPython notebook web application and will display
the fig-nbDashboard as a page in your default web browser.
To create a new IPython notebook, click the New Notebook button. That
will open a new fig-ipyNotebook with the provisional title Untitled0
in a new tab in your browser. To give the notebook a more meaningful
name, click on Untitled0 in your browser window to the right of
IP[y]: Notebook and rename your notebook FirstNotebook. The Name
FirstNotebook will replace Untitled0 in your Notebook browser window
and a file named FirstNotebook.ipynb will appear in the directory from
which you launched IPython Notebook. That file will contain all the work
you do in the IPython notebook. Next time you launch IPython Notebook
from this same directory, all the IPython notebooks in that directory
will appear in a list on the IPython Notebook Dashboard. Clicking on one
of them will launch that notebook.
When you open a new IPython Notebook, an IPython interactive cell with
the prompt In[ ]: to the left, appears. You can type code into this
cell just as you would in the IPython shell of the fig-AnacondaWindow.
For example, typing 2+3 into the cell and pressing Shift-Enter (or
Shift-Return) to execute the cell yields the expected result. Try it
out.
Below the result a new IPython interactive cell appears ready for your
next entry. In the next cell type the commands to import numpy and
matplotlib.pyplot as shown in the fig-firstIPyNBcells figure. These
two commands are separated by the Shift (without Enter or Return)
so that they appear in the same cell. They are then both executed by
pressing Shift-Enter (or Shift-Return). After importing numpy,
typing sin(np.pi/6.) and pressing Shift-Enter produces the expected
result (to nearly 1 part in $10^{16}$).
You can also create plots in an IPython notebook. For example, typing
plt.plot([1,2,3,2,3,4,3,4,5]) and pressing Shift-Enter produces the
same plot shown in the fig-zigzagPlotDemo figure. The plot is produced
"in line" and not in a separate window, because we used the
--matplotlib inline switch when we launched IPython Notebook. If you
have followed along, your IPython notebook should look something like
that shown in the figure fig-firstIPyNBcells.
When importing NumPy and MatPlotLib, you can import the entire libraries
using the commands "from numpy import *" and
"from matplotlib.pyplot import *". Importing the two libraries in this
way means that you don't have to use the np. and plt. prefixes when
calling NumPy and MatPlotLib functions, just as in the IPython console
in Anaconda or Spyder. While this could lead to some confusion in the
namespaces, it's usually harmless.
Be sure to press the Save and Checkpoint icon at the far left near the
top of the IPython Notebook window from time to time to save your
work.
You can also run programs in an IPython notebook. As an example, we run
the program introduced in the section on screenoutput. The program is
input into a single notebook cell and then executed by pressing
Shift-Enter.
In this example, the program requests input from the user: the distance
of the trip. The program runs up to the point where it needs input from
the user, and then pauses until the user responds and presses the
Enter or Return key. The program then completes its execution. Thus
the IPython notebook provides a complete log of the session.
An IPython notebook will be more easily comprehended if it includes annotations of the session. In addition to logging the inputs and outputs of computations, IPython Notebook allows the user to embed headings, explanatory notes, mathematics, and images.
Suppose, for example, that we want to have a title at the top of the
IPython notebook we have been working with, and we want to include the
name of the author of the session. To do this, we scroll the IPython
notebook back up to the top and place the cursor and click in the very
first input cell, the one that contained 2+3. We then open the
Insert menu near the top center of the window and click on
Insert Cell Above, which opens up a new input cell above the first
cell. Next, we click on the box in the Toolbar that says Code. A list
of cell types appears: Code (currently checked), Markdown,
Raw Text, Heading 1, Heading 2, ..., Heading 6. Select
Heading 1; immediately the In [ ]: prompt disappears, indicating
that this box is no longer meant for inputing and executing Python code.
Type "Demo of IPython Notebook" and press Shift-Enter (or
Shift-Return). A heading in large print appears before the first
IPython code cell. Place the cursor back in the first Ipython code cell
(2+3). Once again, select the Insert menu and click on
Insert Cell Above. Again, click on the Toolbar that says Code, but
this time select Heading 2. Type your name into the newly created cell
and press Shift-Enter. Your name is printed in the cell in slightly
smaller print than for the previous case.
You can also write comments, including mathematical expressions, into an
IPython Notebook cell. Let's include a comment after the program we ran
that calculated the cost of gasoline for a road trip. First we place the
cursor in the open formula cell below program we ran and then click on
the box in the Toolbar that says Code and change it to Markdown.
Returning to the cell, we enter the text of our comment. We can enter
any text we wish, including mathematical expressions using the markup
language Latex. (If you do not already know Latex, you can get a brief
introduction at these sites:
http://en.wikibooks.org/wiki/LaTeX/Mathematics or
ftp://ftp.ams.org/pub/tex/doc/amsmath/short-math-guide.pdf.) Here we
enter the following text:
The total distance $x$ traveled during a trip can be obtained by integrating the velocity $v(t)$ over the duration $T$ of the trip: \begin{align} x = \int_0^T v(t)\, dt \end{align}
After entering the text, pressing Shift-Enter yields the result shown
in fig-markdown.
The $ symbol brackets inline mathematical expressions in Latex, while
the \begin{align} and \end{align} expressions bracket displayed
expressions. You only need to use Latex if you want to have fancy
mathematical expressions in your notes. Otherwise, they are not
necessary.
Suppose you were importing a data (.txt) file from your hard disk and
you wanted to print it out in one of the notebook cells. If you were in
the Terminal (Mac) or Command Prompt (PC), you could write the
contents of any text file using the command cat filename (Mac) or
type filename (PC). You can execute the same operation from the
IPython prompt using the Unix (Mac) or DOS (PC) command preceded by an
exclamation point, as described in the section on sysCommands.
In working with an IPython notebook, you may find that you want to move
some cells around, or delete some cells, or simply change some cells.
All of these tasks are possible. You can cut and paste cells, as in a
normal document editor, using the Edit menu. You can also freely edit
cells and re-execute them by pressing Shift-Enter. Sometimes you may
find that you would like to re-execute the entire notebook afresh. You
can do this by going to the Kernel menu and selecting Restart. A
warning message will appear asking you if you really want to restart.
Answer in the affirmative. Then open the Cell menu and select
Run All, which will re-execute the notebook starting with the first
cell. You will have to re-enter any screen input requested by the
notebook scripts.
It goes almost without saying that before quitting an IPython notebook,
you should make sure you have saved the notebook by pressing the
Save and Checkpoint item in the File menu or its icon in the
Toolbar.
When you are ready to quit working with a notebook, click on the
Close and halt item in the File menu. Your notebook browser tab will
close and you will return to the IPython Notebook Dashboard. Just close
the IPython Notebook Dashboard tab in your browser to end the session.
Finally, return to the Terminal or Command Prompt application, hold
down the control key and press c twice in rapid succession. This
stops the IPython Notebook session. You should see the normal system
prompt. You can then close the Terminal (Mac) or Command Prompt (PC)
session if you wish.
To work with an existing IPython notebook, open the Terminal (Mac) or
Command Prompt (PC) application and navigate to the directory in which
the notebook you want to work with resides. Recall that IPython
notebooks have the .ipynb extension. Launch the IPython Notebook
Dashboard as you did previously by issuing the command
ipython notebook --matplotlib inline &
This will open the IPython Notebook Dashboard in your web browser, where you should see a list of all the IPython notebooks in that directory (folder). Click on the name of the notebook you want to open. It will appear in a new tab on your web browser as before.
Note that while all the input and output from the previous saved session
is present, none of it has been run. That means that none of the
variables or other objects has been defined in this new session. To
initialize all the objects in the file, you must rerun the file. To
rerun the file, press the Cell menu and select Run All, which will
re-execute all the cells. You will have to re-enter any screen input
requested by the notebook scripts. Now you are ready to pick up where
you left off the last time.