# Source code for sympy.printing.printer

"""Printing subsystem driver

SymPy's printing system works the following way: Any expression can be
passed to a designated Printer who then is responsible to return an

The basic concept is the following:
1. Let the object print itself if it knows how.
2. Take the best fitting method defined in the printer.
3. As fall-back use the emptyPrinter method for the printer.

Some more information how the single concepts work and who should use which:

1. The object prints itself

This was the original way of doing printing in sympy. Every class had
its own latex, mathml, str and repr methods, but it turned out that it
is hard to produce a high quality printer, if all the methods are spread
out that far. Therefor all printing code was combined into the different
printers, which works great for built-in sympy objects, but not that
good for user defined classes where it is inconvenient to patch the
printers.

Nevertheless, to get a fitting representation, the printers look for a
specific method in every object, that will be called if it's available
and is then responsible for the representation. The name of that method
depends on the specific printer and is defined under
Printer.printmethod.

2. Take the best fitting method defined in the printer.

The printer loops through expr classes (class + its bases), and tries
to dispatch the work to _print_<EXPR_CLASS>

e.g., suppose we have the following class hierarchy::

Basic
|
Atom
|
Number
|
Rational

then, for expr=Rational(...), in order to dispatch, we will try
calling printer methods as shown in the figure below::

p._print(expr)
|
|-- p._print_Rational(expr)
|
|-- p._print_Number(expr)
|
|-- p._print_Atom(expr)
|
-- p._print_Basic(expr)

if ._print_Rational method exists in the printer, then it is called,
and the result is returned back.

otherwise, we proceed with trying Rational bases in the inheritance
order.

3. As fall-back use the emptyPrinter method for the printer.

As fall-back self.emptyPrinter will be called with the expression. If
not defined in the Printer subclass this will be the same as str(expr).
"""

from __future__ import print_function, division

[docs]class Printer(object): """Generic printer Its job is to provide infrastructure for implementing new printers easily. Basically, if you want to implement a printer, all you have to do is: 1. Subclass Printer. 2. Define Printer.printmethod in your subclass. If a object has a method with that name, this method will be used for printing. 3. In your subclass, define _print_<CLASS> methods For each class you want to provide printing to, define an appropriate method how to do it. For example if you want a class FOO to be printed in its own way, define _print_FOO:: def _print_FOO(self, e): ... this should return how FOO instance e is printed Also, if BAR is a subclass of FOO, _print_FOO(bar) will be called for instance of BAR, if no _print_BAR is provided. Thus, usually, we don't need to provide printing routines for every class we want to support -- only generic routine has to be provided for a set of classes. A good example for this are functions - for example PrettyPrinter only defines _print_Function, and there is no _print_sin, _print_tan, etc... On the other hand, a good printer will probably have to define separate routines for Symbol, Atom, Number, Integral, Limit, etc... 4. If convenient, override self.emptyPrinter` This callable will be called to obtain printing result as a last resort, that is when no appropriate print method was found for an expression. Examples of overloading StrPrinter:: from sympy import Basic, Function, Symbol from sympy.printing.str import StrPrinter class CustomStrPrinter(StrPrinter): \"\"\" Examples of how to customize the StrPrinter for both a SymPy class and a user defined class subclassed from the SymPy Basic class. \"\"\" def _print_Derivative(self, expr): \"\"\" Custom printing of the SymPy Derivative class. Instead of: D(x(t), t) or D(x(t), t, t) We will print: x' or x'' In this example, expr.args == (x(t), t), and expr.args[0] == x(t), and expr.args[0].func == x \"\"\" return str(expr.args[0].func) + "'"*len(expr.args[1:]) def _print_MyClass(self, expr): \"\"\" Print the characters of MyClass.s alternatively lower case and upper case \"\"\" s = "" i = 0 for char in expr.s: if i % 2 == 0: s += char.lower() else: s += char.upper() i += 1 return s # Override the __str__ method of to use CustromStrPrinter Basic.__str__ = lambda self: CustomStrPrinter().doprint(self) # Demonstration of CustomStrPrinter: t = Symbol('t') x = Function('x')(t) dxdt = x.diff(t) # dxdt is a Derivative instance d2xdt2 = dxdt.diff(t) # dxdt2 is a Derivative instance ex = MyClass('I like both lowercase and upper case') print dxdt print d2xdt2 print ex The output of the above code is:: x' x'' i lIkE BoTh lOwErCaSe aNd uPpEr cAsE By overriding Basic.__str__, we can customize the printing of anything that is subclassed from Basic. """ _global_settings = {} _default_settings = {} emptyPrinter = str printmethod = None def __init__(self, settings=None): self._str = str self._settings = self._default_settings.copy() for key, val in self._global_settings.items(): if key in self._default_settings: self._settings[key] = val if settings is not None: self._settings.update(settings) if len(self._settings) > len(self._default_settings): for key in self._settings: if key not in self._default_settings: raise TypeError("Unknown setting '%s'." % key) # _print_level is the number of times self._print() was recursively # called. See StrPrinter._print_Float() for an example of usage self._print_level = 0 @classmethod