Sage FAQ: Frequently Asked Questions
Contents
 Introduction
 Getting Sage

Installing and running Sage
 Wouldn't it be way better if Sage did not ship as a gigantic bundle?
 How do I compile the source to Sage?
 How do I run Sage on a platform other than VirtualBox or Windows?
 How do I run Sage with VirtualBox?
 How do I create my own Sage virtual machine ?
 How do I run Sage in Windows?
 I installed Sage on a Unix or Linux server where I have an account, but my personal computer runs Windows. How do I access the Sage notebook?
 How do I run a parallel build?
 How do I run Sage in a browser that is not the system default
 How to get Sage's Python to recognize my system's Tcl/Tk install?
 How do I import Sage into a Python script?
 I'm seeing an error about 'Permission denied' on a file called sageflags.txt
 I used Debian/Ubuntu to install Sage 3.0.5 and that version is giving lots of errors. What can I do?
 Can I see a calculation stepbystep?
 Developing in Sage

Working in Sage
 What exactly does Sage do when I type 0.6**2?
 Why is Sage's command history different than Magma's
 How do I work with noncommutative variables
 How do I save an object so I don't have to compute it each time I open a worksheet?
 I get an error from jsMath or the math symbols don't look right when displaying in the notebook
 Getting help
 Other questions
1. Introduction
1.1. What is Sage?
Sage is a comprehensive opensource mathematics software suite that has the mission statement "Creating a viable free open source alternative to Magma, Maple, Mathematica, and Matlab." See http://www.sagemath.org/ for more details.
1.2. Does it run on my computer?
Probably, we are trying to support a wide range of computers. See the supported platforms list for details.
2. Getting Sage
2.1. Can I try out Sage without downloading anything?
Yes! Go to http://www.sagenb.org/ and set up a free account. If you log in, you will be working on a free Sage notebook server that will work identically to the one you get with Sage.
2.2. How do I get a Sage program I can run immediately?
Go to http://www.sagemath.org/download.html and click on the link for the binary for your operating system.
2.3. How do I get the Sage source code?
Go to http://www.sagemath.org/downloadsource.html to download the tar archive for any release of Sage.
2.4. How do I get a previous release of Sage?
Go to http://www.sagemath.org/srcold to download the tar archive for any release of Sage.
3. Installing and running Sage
3.1. Wouldn't it be way better if Sage did not ship as a gigantic bundle?
This has been discussed over and over again and it plainly doesn't work.
3.2. How do I compile the source to Sage?
Download the source tar archive, extract the archive, change your directory to be inside of it, and read the README.txt file there. Basically, after making sure you have the proper prerequisite tools installed, you type make.
3.3. How do I run Sage on a platform other than VirtualBox or Windows?
Change your directory to the sage directory and run ./sage
To start an online notebook server, start Sage and type notebook() at the sage command prompt.
3.4. How do I run Sage with VirtualBox?
You must install the VirtualBox software available at http://www.virtualbox.org/wiki/Downloads. Then run the downloaded Sage virtual appliance and follow these additional instructions.
3.5. How do I create my own Sage virtual machine ?
See these instructions.
3.6. How do I run Sage in Windows?
Windows is currently supported via the VirtualBox image, so see the instructions for running the Sage virtual appliance. See windows for information on efforts to make a native port of Sage to Windows.
3.7. I installed Sage on a Unix or Linux server where I have an account, but my personal computer runs Windows. How do I access the Sage notebook?
You can use port forwarding with ssh to interact with the notebook via a Windows browser.
Log into the server, and run the Sage notebook. Typically this involves the command
./sage notebook
from within the Sage directory.
If this is the first time you have used the Sage notebook, the system will prompt you to create an admin password. The system will then give you instructions that look like this:
************************************************** * * * Open your web browser to http://localhost:8080 * * * **************************************************
Make a note of the number after localhost (in our example, 8080). This is the port that the notebook expects you to use.
Now, close the notebook using CtrlC, and log out of the server. You want to open a new ssh connection using port forwarding.
If you are using the free ssh program PuTTy, you should first enter the name of your server. Then find the port forwarding options: you can locate these by going to Connections, then SSH, then Tunneling in the "Category" menus on the left. Enter the port number (e.g., 8080) in the Source Port box, and then enter localhost: followed by the port number (e.g., localhost:8080) in the Destination box. Make sure the Local radio button is selected, and click Add. Then hit Open to log into your server.
Return to your sage directory, and run
./sage notebook
again.
This time, when the notebook prompts you to open your web browser to a particular address, you should do so (in our example, that means opening a new browser window and going to http://localhost:8080 ). Log in using the username admin and the password you created. Once you have done so, you can create new users by following the Settings link and then choosing the Manage Users option.
3.8. How do I run a parallel build?
The command
export MAKE="make j8"
will enable 8 threads for parts of the build that support parallelism. Change the number 8 as appropriate to suit the number of cores on your system.
The command
export SAGE_PARALLEL_SPKG_BUILD="yes"
will enable building the spkgs in parallel.
3.9. How do I run Sage in a browser that is not the system default
Issue this command "env SAGE_BROWSER=opera /usr/bin/sage notebook" either from the command prompt or as a menu command for Sage. Assumes a Linux operating system, Opera as the browser, and I happen to use KDE as my desktop.
3.10. How to get Sage's Python to recognize my system's Tcl/Tk install?
It may be that you have Tcl/Tk installed and that your system's Python recognizes it but Sage's Python does not. To fix that, install the tcl/tk development library. On Ubuntu, this is the command
sudo aptget install tk8.5dev
Next, reinstall Sage's Python:
sage f python2.5.2.p8
This will pick up the tcl/tk library automatically. If
sage: import _tkinter
sage: import Tkinter
does not raise an ImportError then it worked.
3.11. How do I import Sage into a Python script?
Yes you can import Sage as a library in a Python script. One caveat is that you need to run that Python script using the version of Python that is bundled with Sage (e.g. Sage 5.8 has Python 2.7.3). To import Sage, put the following in your Python script:
from sage.all import *
Then when you want to run your script, you need to invoke Sage with the option "python" which would run your script using the Python that comes with Sage. For example, if Sage is in your PATH variable then you can do this:
sage python /path/to/my/script.py
Another way is to write a Sage script and run that script using Sage itself. A Sage script has the file extension ".sage" and is more or less a Python script but uses Sagespecific functions and commands. You can then run that Sage script like so:
sage /path/to/my/script.sage
This will take care of loading the necessary environment variables and default imports for you.
3.12. I'm seeing an error about 'Permission denied' on a file called sageflags.txt
When sage is built from source, it keeps track of what special instructions your CPU supports (such as SSE2), and records these (so that if you try running the code on a different machine, which doesn't support these extra instructions, you get a sensible error message instead of a segfault or illegal instruction). Since this should be stored with sage itself (as opposed to a user's .sage directory), it has to be created by someone with the appropriate permissions.
So if you're seeing something like this:
Traceback (most recent call last):
File "/usr/local/sage4.0.2/local/bin/sagelocation", line 174, in <module>
t, R = install_moved()
File "/usr/local/sage4.0.2/local/bin/sagelocation", line 18, in install_moved
write_flags_file()
File "/usr/local/sage4.0.2/local/bin/sagelocation", line 82, in write_flags_file
open(flags_file,'w').write(get_flags_info())
IOError: [Errno 13] Permission denied: '/usr/local/sage4.0.2/local/lib/sageflags.txt'
it probably means that you compiled/installed sage as one user, but haven't run it to let it generate the sageflags.txt file. Just run sage one time as whatever user installed it, and this problem should go away. This would also be easy to fix by having sage run once as part of the install process; see trac #6375 for this fix.
3.13. I used Debian/Ubuntu to install Sage 3.0.5 and that version is giving lots of errors. What can I do?
The version of Sage, i.e. Sage version 3.0.5, that's available through aptget in Debian and Ubuntu is very old. Noone has yet found time to update the Debian/Ubuntu version of Sage. Help wanted! You should download the latest version of Sage from the download page. If you would like to help with updating the Debian/Ubuntu version of Sage, please email the sagedevel mailing list.
3.14. Can I see a calculation stepbystep?
Unfortunately no. You can run the python debugger to execute it stepbystep, but that is entirely different from how a human would do a calculation.
4. Developing in Sage
4.1. What tools do I need to develop in Sage?
You need the prerequisite tools listed in the ([https://github.com/sagemath/sage/blob/master/README.txtREADME.txt]] file in the root directory of Sage.
4.2. Where is the source code to Sage?
You can browse the complete source code to everything in Sage at https://github.com/sagemath/sage. This is a web interface to the Git repository.
4.3. How can I rebuild the Sage library in parallel?
When you run sage b, the Sage library gets rebuilt. If you've changed a large number of files (for example, by using Mercurial to jump over many revisions), rebuilding the library will take a long time. If you are using a multicore machine, you can speed this up very easily: set the MAKE environment variable. If you run, say, env MAKE='make j5' ./sage b then the Sage library will get rebuilt using five threads. The other libraryrebuilding commands also work: br, ba, and baforce all respect MAKE. In the bash shell, you can also simply do export MAKE='make j5' and thereafter any rebuilds you do will automatically use five threads.
Note that there is an apparent bug in GNU make, so that you cannot leave a space between the j and the number of threads (jobs, as Make calls them). So make j 5 and so on will not work.
5. Working in Sage
5.1. What exactly does Sage do when I type 0.6**2?
 QUESTION: When I type 0.6**2 in Python, it returns 0.35999999999999999. When I do the same in Sage it returns 0.360000000000000. Why?
ANSWER: See the Python tutorial to understand why Python does what it does. What Sage does is "preparse" the input and transforms it like this:
sage: preparse('0.6**2') "RealNumber('0.6')**Integer(2)"
So what is *actually* run is:
RealNumber('0.6')**Integer(2)
The Sage developers (in fact, Carl Witty) decided that Sage floating point numbers should by default print only the known correct decimal digits, when possible, thus skirting the problem that Python has. This decision has its pros and cons. Note that RealNumber and Integer are Sage specific, so you wouldn't be able to just type the above into python and expect it to work without first doing from sage.all import RealNumber, Integer, preparse.
5.2. Why is Sage's command history different than Magma's
 QUESTION: Using Sage, I am missing a feature of Magma command line interface. In Magma, if I enter a line found in history using up arrow key, and then press down arrow key, then the next line in history is fetched. This feature allows me to fetch as many successive lines in history as like. Does Sage(or readline) have a similar feature?
 ANSWER: No, Sage does not have a similar feature. The IPython command prompt uses the readline library (via pyreadline), which evidently doesn't support this feature. Magma has its own custom "readlinelike" library, which does support this feature. (Since so many people have requested this feature, if anybody can figure out how to implement it, then such an implementation would certainly be welcome!)
5.3. How do I work with noncommutative variables
 QUESTION: I'd like to do some mathematics with variables that do not commute with each other. How do I do that?
ANSWER: Use the FreeAlgebra object:
sage: R.<a,b> = FreeAlgebra(QQ, 2) sage: a*b + b*a a*b + b*a sage: s = matrix(R) sage: s = matrix(R, [[a,b],[b,a]]); s [a b] [b a] sage: s*s [a^2 + b^2 a*b + b*a] [a*b + b*a a^2 + b^2] sage: b*s*s [ b*a^2 + b^3 b*a*b + b^2*a] [b*a*b + b^2*a b*a^2 + b^3]
For matrices in particular, see http://groups.google.com/group/sagedevel/browse_thread/thread/cafb486c79a2eb3c/d0bb78d09a4fb52a
5.4. How do I save an object so I don't have to compute it each time I open a worksheet?
The save and load commands will save and load an object, respectively. In the notebook, the DATA variable is the location of the data storage area of the worksheet. To save the object my_stuff in a worksheet, you could do save(my_stuff, DATA+"my_stuff") and to reload it, you would just do my_stuff = load(DATA+"my_stuff")
5.5. I get an error from jsMath or the math symbols don't look right when displaying in the notebook
If you see the error "It looks like jsMath failed to set up properly (error code 7). I will try to keep going, but it could get ugly.", you haven't installed the TeX fonts which help jsMath render beautiful mathematics. To get the nice TeX display with jsMath, please download a set of fonts from here:
http://www.math.union.edu/~dpvc/jsMath/download/jsMathfonts.html
If you are on Linux/Unix, ignore the instructions on the page and just unzip the fonts into your ~/.fonts directory. You can also install the "jsmathfonts" package.
6. Getting help
6.1. How do I get help?
There is a website for asking help: http://ask.sagemath.org.
Sage has two very active email lists: http://groups.google.com/group/sagedevel and http://groups.google.com/group/sagesupport.
There are also two very active IRC channels: #sagedevel and #sagesupport on freenode.
Many developers also actively blog and also post other Sagerelated tutorials and talks. See http://www.sagemath.org/help.html for a listing of these resources.
7. Other questions
QUESTION: I downloaded a Sage binary and it crashes on startup with Illegal instruction. What can I do?
 ANSWER: Short answer: One way to fix this is to build Sage entirely from source. Another option is to fix your Sage installation by rebuilding MPIR and ATLAS by typing the following in the root of your Sage installation directory and wait about 1520 minutes:
rm spkg/installed/mpir* spkg/installed/atlas* make
More explanation: The binaries have been built for a newer architecture than what you have. Nobody has yet figured out how to build Sage in such a way that MPIR and ATLAS work on all hardware. This will eventually get fixed.
QUESTION: I created the file SAGE_ROOT/devel/sage/sage/calculus/stokes.py, and have changed my mind and want to completely delete it from Sage, but it keeps coming back (i.e. it is still importable) when I type sage br. What do I do?
ANSWER: Delete both SAGE_ROOT/devel/sage/build/sage/calculus/stokes.py and SAGE_ROOT/devel/sage/build/lib.*/sage/calculus/stokes.py.
QUESTION: Does sage contain a function similar to Mathematica's ToCharacterCode[]? That is, I'd like to covert something like "Big Mac" to ascii numerals for further processing. Thanks
ANSWER: Yes, use ord, e.g., map(ord,'abcde') outputs [97, 98, 99, 100, 101].
 QUESTION: Is there anything so Sage can be made to automatically execute commands on startup?
ANSWER: Yes, just make a file $HOME/.sage/init.sage and it will be executed any time you start sage.
 QUESTION: My Sage upgrade failed with missing gmp symbols on OSX 10.4. What can I do?
 ANSWER: Moving a sage install on OSX 10.4 and then upgrading anything that is linked against NTL leads to link errors due to missing gmp symbols. The problem is the link mode with which the dynamic NTL is created. I have a fix, but I am currently verifying that it really fixes the issue. Everything that is linked against NTL needs to be recompiled, i.e. singular and cremona at the moment. To add to the confusion: This is not an issue on OSX 10.5. A fix for this issue went into 2.8.15, so please report if you see this with a more current Sage release.
 QUESTION: When I compile Sage my computer beeps and shuts down or hangs.
 ANSWER 1: Compiling Sage is quite taxing on the CPU. The above behavior usually indicates that your computer has overheated. In many cases this can be fixed by cleaning the CPU fan and assuring proper ventilation of the system. Please ask your system administrator or a professional to do this in case you have never done this since you can potentially damage your system.
 ANSWER 2: For Linux users, if you suspect that the compilation fails because of a resource issue, a fix might be to edit your /etc/inittab so that Linux boots into run level 3. The file /etc/inittab usually contains something similar to the following snippet:
# 0  halt (Do NOT set initdefault to this) # 1  Single user mode # 2  Multiuser, without NFS (The same as 3, if you do not have networking) # 3  Full multiuser mode # 4  unused # 5  X11 # 6  reboot (Do NOT set initdefault to this) # id:5:initdefault:
which directs your Linux distribution to boot into a graphical login screen. Comment out the line "id:5:initdefault:" and add the line "id:3:initdefault:", so that you now have something like:# 0  halt (Do NOT set initdefault to this) # 1  Single user mode # 2  Multiuser, without NFS (The same as 3, if you do not have networking) # 3  Full multiuser mode # 4  unused # 5  X11 # 6  reboot (Do NOT set initdefault to this) # # id:5:initdefault: id:3:initdefault:
Now if you reboot your system, you'll be greeted with a text based login screen. This allows you to log into your system with a text based session from within a virtual terminal, which doesn't consume as much system resources as would be the case with a graphical session. Then build your Sage source distribution from within your text based session.
 QUESTION: When I run doctests on OSX I see the following messages, but in the end Sage reports that everything went fine:
sage t devel/sagemain/sage/libs/pari/gen.pyx python(4563) malloc: *** vm_allocate(size=4096000000) failed (error code=3) python(4563) malloc: *** error: can't allocate region python(4563) malloc: *** set a breakpoint in szone_error to debug
 ANSWER: The issue above isn't a doctest failure, it is an error message printed by the system and it is exactly what one expects to see  in that particular doctest, we try to allocate a very large list in Pari that doesn't fit into physical memory (it is at least 100GB in size). So OSX tells you that it couldn't allocate a chunk of memory roughly 4 GB in size, which is expected, since Sage is still a 32 bit application on OSX.
 QUESTION: Sage 2.9 and higher fails compiling ATLAS on Linux. How can I fix this?
ANSWER: The most likely cause is enabled power management. Disabling it should fix the problem. Depending on your flavor of distribution, this might either be possible with some nice GUI tool or not. On the command line do the following as root for each CPU you have: /usr/bin/cpufreqselector g performance c #number CPU. On Ubuntu try disabling "Power Manager" via "System > Preferences > Sessions" under the "Startup Programs" or using cpufreqset via command line.
QUESTION: Sage fails with the error message restore segment prot after reloc: Permission denied. What is wrong?
ANSWER: The problem is related to SELinux: http://www.ittvis.com/services/techtip.asp?ttid=3092. We are currently tracking this issue at http://trac.sagemath.org/sage_trac/ticket/480.
 QUESTION: When I start Sage, SELinux complains that '/path/to/libparigmp.so.2' requires textrelocation. How can I fix it?
 ANSWER: The problem can be fixed by running the following command "chcon t textrel_shlib_t /path/to/libparigmp.so.2"
 QUESTION: Upgrading Sage went fine, but now the banner still shows the old version. How can I fix this?
ANSWER: You can type sage: banner() to see the real version. You have to to type make build to rebuild sage and get the correct banner.
 QUESTION: How do I run sage in daemon mode, i.e. as a service?
ANSWER: We currently do not have a readytogo solution. There are several possibilities: Use screen, nohup or disown. We are tracking the issue at http://trac.sagemath.org/sage_trac/ticket/381  so stay tuned.
QUESTION: I just downloaded version 2.8.15 for Mac OSX and tried to run notebook() and dyld is unable to load libintl.3.dylib. I don't have a libintl.3.dylib in usr/local/lib and I didn't find it in $SAGE_ROOT/local/lib. Is there a workaround?
ANSWER: Yes, put the libintl3.dylib from http://sagemath.org/SAGEbin/apple_osx/intel/10.4extra_files/ in SAGE_ROOT/local/lib/. Sage 2.8.15 and later contain a copy of the library, so please let us know if you experience the problem with any later release.
 QUESTION: I am using Mac OSX. Where do I put the jsMath "font" directory to eliminate the red box?
ANSWER: See http://www.math.union.edu/~dpvc/jsMath/download/jsMathfonts.html where it says "For Mac OS X users: download and unpack the archive, then drag the fonts to your Library/Fonts folder (or to the FontBook, or just doubleclick them and press the "install" button).".
 QUESTION: The show command for plotting 3D objects does not work.
 ANSWER 1: Since Sage 2.9.2 we have switched to using jmol, a Java applet, for 3D plotting. There are several possibilities for the cause of the malfunction: You do not have Java installed at all or the Java installed is an older GNU based alternative Java implementation, which causes some yet to determine problem. A solution to both issues is to either install Sun's Java SDK or to update the GNU based Java implementation. As of January 2008 Debian's Java in testing works, but stable does have problems.
ANSWER 2: If you are running a brand new (as of April 2008) Ubuntu 8.04, they ship the Java Plugin by IcedTea. This is basically a good idea, but a bit too early since it is broken. Either wait for an update or uninstall the IcedTea Plugin and install the "SUN Java 6 Plugin". Later, switch back to IcedTea, since it is based on OpenJDK 7 (or SUNs Java 7) which is the next Java version. You can check for the used plugin in Firefox 3 by typing "about:plugins" into the URL bar. read more here at launchpad
 QUESTION: May I use Sage tools in a commercial environment?
 ANSWER: YES! Absolutely! Basically the *only* constraint is that if you make changes to Sage itself and redistribute this changed version of Sage publicly, then you must make these changes available to us so that we can put them into the standard version of Sage (if we want). Otherwise, you are free to use as many copies of Sage as you want completely for free to make money, etc., without paying any license fees at all.
QUESTION: I want to write some Cython code that uses finite field arithmetic but cimport sage.rings.finite_field_givaro fails. What can I do?
 ANSWER: You need to give hints to Sage so that it uses C++ (both Givaro and NTL are C++ libraries), and it also needs the GMP and STDC C++ libraries. Here is a small example:
# These comments are hints to Sage/Pyrex about the compiler and # libraries needed for the Givaro library: # #clang c++ #clib givaro gmpxx gmp m stdc++ cimport sage.rings.finite_field_givaro # Construct a finite field of order 11. cdef sage.rings.finite_field_givaro.FiniteField_givaro K K = sage.rings.finite_field_givaro.FiniteField_givaro(11) print "K is a", type(K) print "K cardinality =", K.cardinality() # Construct two values in the field: cdef sage.rings.finite_field_givaro.FiniteField_givaroElement x cdef sage.rings.finite_field_givaro.FiniteField_givaroElement y x = K(3) y = K(6) print "x is a", type(x) print "x =", x print "y =", y print "x has multiplicative order =", x.multiplicative_order() print "y has multiplicative order =", y.multiplicative_order() print "x*y =", x*y # Show that x behaves like a finite field element: for i in range(1, x.multiplicative_order() + 1): print i, x**i assert x*(1/x) == K.one_element()
To find out more, type sage.rings.finite_field_givaro.FiniteField_givaro. at the Sage prompt and hit tab, then use ?? to get more information on each function. For example: sage.rings.finite_field_givaro.FiniteField_givaro.one_element?? tells you more about the multiplicative unit element in the finite field.
# * QUESTION: How do I make the VMware appliance for Windows automatically login as "sage"? # * ANSWER: Follow http://ubuntuutah.ubuntuforums.org/showthread.php?t=303319. Short version: put in the file /usr/bin/autologin the text #
#/bin/login f sage #
# and make this file executable; then edit /etc/event.d/tty1, comment out #
#respawn /sbin/getty 38400 tty1 #
# and add #
#respawn /sbin/getty n 1 /usr/bin/autologin 38400 tty1 #
# Now every time the appliance reboots, it will automatically load directly to the sage: prompt. Warning: This will make it nearly impossible to get a terminal prompt! So only do this if you don't plan on any further management. # #If you do need to escape to a shell, you can run the following from inside sage (untested): #
#import os
#os.execp('sh')
#
#then use "sudo s" to get a root shell. #
# * QUESTION: When running Sage under VMware, if you log in under manage you are not given the permissions to create a file! # * ANSWER: Type "sudo su" before creating files. #
 QUESTION: I'm getting weird build failures on OSX. How do I fix this?
 ANSWER: Search the build log (install.log) to see if you're getting "fork: Resource temporarily unavailable.". If so, try the following. Create (or edit) /etc/launchd.conf and include the following:
limit maxproc 512 2048
then reboot. See this page for more details.
 QUESTION: How do I use the bitwise XOR operator in Sage?
ANSWER: Define two variables, for example a = 5; b = 8, and evaluate a.__xor__(b), 13. You can also do (5).__xor__(8) (the parentheses are necessary so that Sage doesn't think you have a real number). There are several ways to define a function: xor = lambda x, y: x.__xor__(y) and then do xor(3, 8). Another option, which sneaks around the Sage preparser, is def xor(a,b): return eval("%s^%s"%(a,b)). You can also turn off the Sage preparser with preparser(False)  then ^ will work just like in Python, and you can later turn on the preparser with preparser(True). (That only works in commandline Sage; in a notebook, switch to Python mode.)
QUESTION: When I try to use LaTeX in the notebook, it says it cannot find fullpage.sty!
ANSWER: That's not a question. But we can help you solve your problem. The general  but perhaps not very helpful  answer is that you need to install fullpage.sty into a directory searched by TeX. In Ubuntu (and probably many other Linux distributions), you should install the texlivelatexextra package. If that's not available, try installing the tetexextra package. If you are using OS X, you will have to use whatever TeX distribution you use to get fullpage.sty (if you use MacTeX, it's likely already installed). If you are using the VMware image in Windows, you'll need to log into the VMware image and install texlivelatexextra there.
QUESTION: With objects a and b and a function f, I accidentally typed f(a) = b instead of f(a) == b. This returned a TypeError (as expected), but also deleted the object a. Why?
ANSWER: It's because of how functions are defined in Sage using the f(x) = expr notation using the preparser. Also notice that if you make this mistake inside of an if statement, you will get a SyntaxError before anything else goes wrong, so in this case, there is no problem.
ToDo
 QUESTION: Sage fails to compile on OSX 10.4
 ANSWER: Most likely resource issue.
 QUESTION: Notebook doesn't work
 ANSWER: networking issues, firewall blocking, proxy setting screwed up
Individual FAQ entries should be referable by a URL, possibly http://wiki.sagemath.org/faq?someid=something
You can do that with page anchors: http://wiki.sagemath.org/faq?#something. MoinMoin can insert page anchors but it doesn't seem like that facility is turned on for this installation. If we could get that enabled, we could insert anchors for all the questions on this page.