# Hidden features of Sage

This post is a wiki. Anyone with karma >750 is welcome to improve it.

In the spirit of the StackOverflow threads of "hidden" language features, we can use this thread (community wiki) to aggregate useful but little-known features or tricks of Sage. Perhaps these can be collected and added to the documentation in the future.

edit retag close merge delete

Sort by » oldest newest most voted

The function get_systems from sage.misc.citation will tell you the backends used to evaluate some code. For example,

sage: from sage.misc.citation import get_systems
sage: get_systems('sqrt(x)')
['MPFI', 'MPFR', 'GMP']
sage: get_systems('sqrt(3.4)')
['MPFR']

more

Apparently, this question disapeared, i repost it though i was not its original author. Sorry for not making this answer an anonymous wiki as it should be. In the recent versions of askbot, only one answer per user is allowed, so i had to trick by first writing a comment, and then transform it into an answer, but that way i could not make it anonymous :(

( 2017-03-20 10:57:32 -0500 )edit

The function sage_input (sometimes) alows to get the Sage code to reconstruct an object you like.

sage: m = random_matrix(ZZ,3)
sage: m
[-1  1 -1]
[-2 -2 -1]
[ 1 -5  0]


I like that one, how could i reconstruct it ?

sage: sage_input(m)
matrix(ZZ, [[-1, 1, -1], [-2, -2, -1], [1, -5, 0]])

sage: m == eval(str(sage_input(m)))
True


Another example:

sage: e = m.eigenvalues()[0]
sage: sage_input(e)

R.<x> = QQbar[]
QQbar.polynomial_root(AA.common_polynomial(x^3 + 3*x^2 + 8), CIF(RIF(-RR(3.6128878647175449), -RR(3.6128878647175444)), RIF(RR(0))))
sage: sage_input(e, preparse=False)

R = QQbar['x']
x = R.gen()
QQbar.polynomial_root(AA.common_polynomial(x**3 + 3*x**2 + 8), CIF(RIF(-RR(3.6128878647175449), -RR(3.6128878647175444)), RIF(RR(0))))

more

Sorry for not making this answer an anonymous wiki. In the recent versions of askbot, only one answer per user is allowed, so i had to trick by firts writing a comment, and then transform it into an answer, but that way i could not make it anonymous :(

( 2017-03-20 10:47:39 -0500 )edit

This post is a wiki. Anyone with karma >750 is welcome to improve it.

You can clear the screen by simply entering clear, as in a Bash shell.

more

This post is a wiki. Anyone with karma >750 is welcome to improve it.

A very hidden feature of Python!

For faster code using itertools you may delete the reference at the end of the loop

sage: from itertools import combinations
sage:     sage: timeit('for p in combinations(range(18),5): pass')
625 loops, best of 3: 615 µs per loop
sage:     sage: timeit('for p in combinations(range(18),5): del p')
625 loops, best of 3: 322 µs per loop


The reason is that the iterators in itertools recycle the objects they return if they are the only one to reference it! But is it relevant to optimize Python code?

more

Wow, this is really cool! Is there any documentation anywhere for this?

( 2013-06-02 17:51:55 -0500 )edit
1

Actually not ! You have to read the sources of Python to discover that fact. I wrote few months ago to the developer of itertools which told me that it was a minor speedup (if you want speed you will not use Python).

( 2013-06-02 21:16:46 -0500 )edit

This post is a wiki. Anyone with karma >750 is welcome to improve it.

It is possible to delete user-defined variables, and reset Sage variables back to their default:

sage: a = 1 ; a
1
sage: reset()
sage: a
NameError: name 'a' is not defined


It is also possible to reset only a few things:

sage: a = b = c = 1
sage: reset(['a','b'])
sage: c
1

more

This post is a wiki. Anyone with karma >750 is welcome to improve it.

Locally disable the preparser with the suffix r

It's not really disabling the preparser (the input is still preparsed), but telling the preparser not to process some of the (numerical) input by marking this input as raw (by appending the letter r).

sage: type(12)
<type 'sage.rings.integer.Integer'>
sage: type(12r)
<type 'int'>

sage: type(42.42)
<type 'sage.rings.real_mpfr.RealLiteral'>
sage: type(42.42r)
<type 'float'>


Also works for Python complex numbers:

sage: type(1j)
<type 'sage.rings.complex_number.ComplexNumber'>
sage: type(1jr)
<type 'complex'>


It's a bit similar to specifying some strings as raw by prepending an r to '...' or "..." or '''...''' or """..."""; for instance in '\t' the backslash-t produces a tab, but in r'\t' it stays backslash-t.

more

This post is a wiki. Anyone with karma >750 is welcome to improve it.

To know whether Sage is running from the notebook or the command line, use the misc.embedded() function:

sage: misc.embedded()
False

more

This post is a wiki. Anyone with karma >750 is welcome to improve it.

The import_statements() function, which allows to know what import statement should we do to use an object.

sage: import_statements('RDF')
from sage.rings.real_double import RDF

more

By the way, it would be http://www.faqtory.co/sky/ (useful) (if it doesn't already exist) to have a function to render LaTeX code to an image just large enough to fit the expression

( 2016-09-12 04:11:43 -0500 )edit

This post is a wiki. Anyone with karma >750 is welcome to improve it.

The preparser() function which allows to understand differences between Python and Sage parsers, and between .py and .sage files:

sage: 2^2
4
sage: preparser(False)
sage: 2^2
0
sage: 2**2
4


Conversely, the preparse() function tells you how Sage preparses the input:

sage: preparse('1.0+2^2')
"RealNumber('1.0')+Integer(2)**Integer(2)"

more

This post is a wiki. Anyone with karma >750 is welcome to improve it.

Though is general for python, it is always worth reading: Code like a Pythonista

more