PyLogo is a Logo interpreter written in Python. Its implementation is small, and is based on the language as implemented by UCBLogo. The Logo language is a learning language, intended for children for which more "complete" languages aren't appropriate. Many of Logos language design choices are driven by this, and differ from Python.
Because of the design choices a Logo compiler (or translator) is difficult to write -- aspects of the parsing can only be done at runtime. So PyLogo is an interpreter built on top of another interpreter. So it's not that fast, but it's not so bad either. (Some Logo implementations have been faster, like StarLogo or Object Logo, by using complex optimizations and lazy compilation, and of course an interpreter written in C would be faster)
Logo is a simple an elegent language. Its focused much more on mathematics, abstract thought, and accessibility than on software design; Mindstorms by Seymour Papert is a good introduction to the philosophy. Since many people in the Python community have had aspirations for using Python for these goals, Logo seems like an interesting example upon which to think about learning and programming.
I feel programming environments have a widely untapped potential to revolutionize pre-algebra mathematical learning, and abstraction skills in general. What is challenging in the declarative expressions of algebra is easy and intuitive when using imperative programming with names and values. (Boxer is even more concrete, but that's a task for another day)
Python, while elegant in many ways, does not have the simplicity of Logo, or the casual syntax to make it accessible to absolute beginners, who often don't have the keyboarding skills to write proper Python code, the typographic literacy to identify the detailed punctuation used, and lack the patience to build these skills before producing working programs.
Despite these general goals, PyLogo also offers some practical advantages. One of PyLogo's best features is the ease of using Python code from PyLogo code (and to a lesser degree, vice versa). Logo primitives are easy to add. PyGame primitives would be be very interesting.
- Complete Logo language, based on the UCBLogo dialect.
- Errors produce tracebacks of your Logo code.
- Multiple simultaneous turtles (unfortunately not yet compatible with threading, due to the limitations of Tkinter).
- Easily extensible from Python.
- Written 100% in Python (portable, easy to read, etc).
- Not used by many people; hasn't really been tried-in-fire.
- A lot of erroneous, or even syntactically incorrect Logo will create weird error messages, instead of causing readable error messages like it should.
- Python errors aren't handled too elegantly -- they get wrapped in a special LogoError which creates the Logo traceback (which you want), but this can obscure as well as assist. Python tracebacks and Logo tracebacks don't quite go together.
- Not very fast. A tight loop is about 400x lower than Python, about 10x slower than UCBLogo.
- No IDE. No persistence (mmm, pickle), for storing code or incomplete projects. None of that clever stuff that the commercial Logos have. But maybe someday. Alternately, it may make more sense to use other IDE(ish) Python products and try to add Logo interpretation there.
- No quasi-quote.
$ python setup.py install ... $ pylogo Importing pylogo.builtins ??? 1 + 1 2 ??? to square :size to? repeat 4 [ to? fd :size to? rt 90 to? ] to? end Function: square ??? square 100
I've also included a post about Logo, which is a useful description for people knowledgeable about computer programming languages.
MAKE accepts UCBLogo's form (MAKE "var 10) and a more normal, less surprising form (MAKE :var 10). LOCAL, LOCALMAKE works the same way.
FOR works more like Python's for, e.g. FOR :i :some_sequence [block]. It is a special form (hence :i instead of "i). UCBLogo's FOR is more like Pascal (FOR [i start end step] [block]).
PyLogo doesn't have constrained object types. Variables can hold any kind of object. Because of this many of UCBLogo's awkard constructs -- like properties and files -- can be implemented as normal objects (in UCBLogo properties and files get names, and are referred to by these names -- essentially each object type has its own global namespace and is addressable only by name).
PyLogo doesn't distinguish between procedures and functions, i.e., you do not have to use IGNORE to suppress an error when you ignore the result of a function. All procedures return None, like in Python.
Python dictionaries can be used in PyLogo. NEWDICT creates a new dictionary, and ITEM and SETITEM can be used (like ITEM :key :dict or SETITEM :key :dict :value). You can use the KEYS, VALUES, and ITEMS functions to access the appropriate methods on dictionaries.
The special Python variables None, True, and False are available with the (zero-argument) functions NONE, TRUE, and FALSE.
Logo files can be loaded with the LOAD command. You do not need to use the .logo extension with the name. Python modules can also be loaded, if they are in the current directory. Python modules can be loaded from other locations using the IMPORT statement, like IMPORT [path to my module] to load path.to.my.module.
Scoping is slightly different than in most Logos. Barring the use of LOCAL, when you do something like MAKE :VAR 10, we look for a scope where :VAR is defined; if found we change that binding. If it is not found, we set it in the local scope. Many Logos set it in the global scope.
In all cases :VAR means :var, "var or simply var.
- MAKE :VAR <value>
- LOCALMAKE :VAR ...
- LOCAL :VAR <more vars...>
- TO funcname :VAR ... <default/int>
- FOR :VAR <sequence> [block]
- TELL <object> [block] (normal form) or TELL <object> method args.... ASK means the same as TELL (for the object-oriented functionality)
- MAKEATTR <object> :VAR <value>
Graphics are implemented in the logo_turtle.py module, which is in turn built upon the standard Python turtle module. Most basic commands match UCBLogo, and are typical for most Logos.
PENCOLOR can take a color name (like "white), or used like (PENCOLOR 255 255 0) to set RGB colors exactly.
HIDETURTLE (or HT) speeds up graphics a lot. It usually does in Logo implementations, but there's a larger delay in actions in the Python turtle module than is typical elsewhere.
TURTLEWRITE will write text at the turtle's current position. Use lists for multiple words. (TURTLEWRITE [text] 1) will also move the turtle to the end of the text (otherwise the turtle doesn't change position).
To fill in spaces, use STARTFILL, move your turtle, and finish with ENDFILL. The turtle's path will be turned into a closed shape (even if the path doesn't close, and may cross itself).
DISTANCE will find the distance to another turtle, or if you provide it with two arguments (like (DISTANCE :t1 :t2)) will find the distance between the two given turtles.
ALLTURTLES returns a list of all the turtles.
Multiple turtles are supported in PyLogo. The "current" turtle is always stored in the :TURTLE variable. When you run a command like FD 10 you are asking :TURTLE to do this command.
To create a new turtle, use the NEWTURTLE command. This returns a turtle object, so you will almost always want to do MAKE :t NEWTURTLE to save the result.
To control a different turtle, you can use the TELL command, like TELL :t [fd 10]. This creates a new scope where :TURTLE is bound to :t, then evaluates the block.
PyLogo supports multiple threads of processing.
Right now PyLogo does not support threads being used with turtles -- you can only use turtles from the main thread. This sucks. It has to do with Tk (which the turtles use) being single-threaded. Hopefully we can fix this, because it's these two features together that are most fun to use. (tkthread is my attempt to run turtle commands in the main thread, while queuing up requests for turtle operations in other threads -- for some reason it didn't work, though)
The basic threading function is SPAWN. It works like:
??? SPAWN [REPEAT 10 [PRINT "HI WAIT 1]]
This runs [REPEAT 10...] in another thread. It also adds another scope, so that variables can be set locally to the thread. A :THREADNAME variable is set local to the scope as well, which gives a unique name for the thread.
SPAWN takes an optional argument. If two arguments are given, then the second is the code for the thread, and the first argument is a list that is evaluated in the new scope, but still while in the main thread. I can't remember why I put this in, but it seemed really useful at some point.
WAIT waits the given number of seconds (floats work too, of course). Useful if you don't want your thread to go crazy fast.
SYNC can help you time your events, compensating for the speed of the machine. When you run STARTSYNC a special variable is created locally (:SyncTimestamp). Later calls to SYNC :Secs will wait however long is necessary for so that it will be have been at least :Secs seconds since the last SYNC or STARTSYNC command.
An example of SYNC:
STARTSYNC FOREVER [ FD RANDOM 10 RT RANDOM 90 SYNC 1 ]
This moves the turtle once a second, regardless of how long FD and RT take to run.
Some other extra functions:
- ASSERT, ASSERTEQUAL:
- Just like in Python, assert something is true (or that two things are equal).
- Returns a function object given the function name, e.g., FUNCTION "ASSERT returns the ASSERT function.
- Works like (CATCH [block] "LogoError [except block] ...). Most exceptions get wrapped in a LogoError exception, so catching specific exceptions might not work that well yet. Exceptions (like "LogoError in this example) are matched against the exception class name.
- Like Python's getattr, e.g., GETATTR :dict "items (returns a function object, the items method of the dictionary object; you'll need to use CALL to call it)
- Like setattr, e.g., SETATTR :obj "attr :value.
- Returns the timestamp (seconds from the Unix epoc).
- Calls the function object. Takes optional arguments, which are passed to the function. Use like (CALL :func :arg1 :arg2).
PyLogo uses a kind of funny method for accessing objects, that I hope will be natural in the Logo environment.
In each scope there can be a list of "actors", aka objects. Everytime you run a function, each actor (in order) gets a chance to perform the operation. So, if an actor defines an appropriate method then that is used. This way you can add an actor that basically overrides functions; e.g., if a turtle object defines FD and you add that to the actor list then it will catch all those calls. This is applied dynamically, not lexically. That means that when you call other functions, they too will start using the object's methods. This is an extension of the dynamic scoping in concept; just don't be too clever about it, because I'm sure this feature can bite.
There are three basic operations for objects:
- TELL or ASK:
- These are both equivalent. The simple method is TELL <object> [block], which runs the block with the object on the actor list. You can also do TELL <object> method args..., which will call the given method. Unlike with a block, the object must define the method. You can also use ASK <object> :var, which will get an attribute (and the object must have the var attribute).
- MAKEATTR <object> :VAR <value>:
- This sets an attribute on the object. MAKE never does this, even though attributes from an actor object are available to read as variables.
- CREATE <base> "NAME [class def]:
This creates a new class, or a new instance through cloning. The basis for the new object is <base>. The standard base you should start with is :object. If you give a class as the base, then a new class is created. If you give an instance, a new instance is created and its attributes are updated.
The class def is a series of MAKE and TO statements, forming attributes and methods. Inside a method :SELF is available, as are variables for each of the attributes.
Once you've created the object, both a variable and function are added to the scope -- the function creates a new instance, while the variable refers to the object. Right now there is no way to take arguments while creating an instance. Maybe later.
CREATE :object "POINT [ MAKE :x 0 MAKE :y 0 TO distance :otherpoint [ MAKE :distx :x - ASK :otherpoint :x MAKE :disty :y - ASK :otherpoint :y OUTPUT SQRT (:distx*:distx + :disty*:disty) END ]
This section shows how to add new Logo primitives written in Python.
You should probably put all your new primitives in a file. builtins.py and logo_turtle.py are examples.
The basic form is this:
>>> from pylogo.common import logofunc >>> @logofunc(name="override_name", aliases=("other", "names"), ... aware=True, arity=-1, hide=True) ... def func(interp, *args): ...
- Used when the Logo name is not a valid Python name (otherwise the name of the function is used); an example is the AND function.
- Used when there's multiple names for a function (like FD and FORWARD).
- Used if you want access to the interpreter (the interp argument in this example).
- Used when you want to override the arity (by default it is picked up from the positional arguments): -1 means that the function is "greedy" and will take as many arguments as it can (for example, PRINT).
- Used to mean you don't want the function to show up in listings (even though it is available).
When making primitives with aware=True, you gain access to the interpreter object (this is defined in pylogo.interpreter.Interpreter if you want to look at the source). There's a few methods you may wish to use:
- Evaluate the list, returning the value of the last expression.
- Return the function object. You should use interp.apply() to run it.
- .set_function(functionName, func):
- Add the function to the interpreter.
- .apply(func, args):
- Run the function func with the given args (a list or tuple); mostly this just uses the logoAware rule.
- Return the value of the variable. pylogo.common.LogoNameError is raised if the variable is not found.
- .set_variable(var, value):
- Set the variable.
- .set_variable_local(var, value):
- Set the variable in the local scope (setVariable will find the first scope where the variable is defined, and only if the variable is not found in any parent scope will it set it locally).
- Create and return a new scope (which is attached to a new interpreter object). This is how you create a local scope. To use the new scope, you must start calling methods on the object this method returns.
- Return a list of function names.
- Return a list of variable names.
For reference, here are some other free logo implementations (this list isn't very up-to-date):
- Logo Foundation: a good place to start looking at Logo and learning about Logo; but the software list is very out of date.
- UCBLogo, intended as a lowest-common-denominator of Logos, it's a straight forward, pure, and fairly complete implementation of Logo. PyLogo was written with UCBLogo's dialect in mind.
- aUCBLogo, a fork of UCBLogo, with lots of graphics improvements and multithreading.
- MSWLogo, Windows version of UCBLogo, with another offshoot being FMSLogo.
- ProLOGO, written in Prolog, with graphics but no procedures.
- Turtle Tracks: Java implementation of Logo, also based on the UCBLogo dialect. A quite complete implementation, but with no activity for many years (last release in 1999).
- Boxer: an academic Logo, with novel ideas of program representation and IDE. I personally find this language very interesting, and would love to see some of its ideas in PyLogo, or even better in Python itself. The actual project is inactive.
- StarLogo: an academic highly concurrent Logo programming (lots of independent turtles acting simultaneously)
- StarLogoT: an academic Logo language with an emphasis on concurrency and parallelism. Seems to be inactive, with NetLogo taking its place.
- NetLogo: an academic Logo language oriented on concurrency and independent agents. An emphasis on simulation.
- Elica: an academic OO Logo, with strong graphics capabilities. Only works on Windows.
- Logo in Scheme: a Logo to Scheme compiler (written by me).
- TinyLogo: Logo for the Palm.
- rLogo: Logo that can be distributed in applet form, with several translations.
- XLogo, Mac OS X Logo (incomplete language?)
- Logo nyelv, Hungarian, written in Turbo Pascal,
- JavaLogo, appears to be turtle graphics, without the Logo language.
- Galapago: Java turtle graphics with an incomplete Logo language, focused on Fractals.
- Tortue, written in Java (incomplete language?)
- KLogo-Turtle: KDE turtle graphics, incomplete Logo language.
- Logo++: a C++ Logo-like language (with significant language changes)