PTL: Python Template Language

PTL is the templating language used by Quixote. PTL inverts the usual model used by web templating languages -- embed a real programming language in HTML -- by merely tweaking Python to make it easier to generate HTML pages (or other forms of text). In other words, PTL is basically Python with a novel way to specify function return values. Specifically, PTL has one extra keyword -- template -- and the value of expressions inside templates are kept, not discarded. Here's a sample template:

template foo (x, y = 5):
    "This is a chunk of static text."
    greeting = "hello world" # statement, no PTL output
    print 'Input values:', x, y
    z = x + y
    """You can plug in variables like x (%s)
in a variety of ways.""" % x

    "Whitespace is important in generated text.\n"
    "z = "
    ", but y is "

Templates are the PTL analogue to Python functions. Templates are defined using the template keyword, obviously, and they can't have docstrings, but otherwise they follow Python's syntactic rules: indentation indicates scoping, single-quoted and triple-quoted strings can be used, the same rules for continuing lines apply, and so forth. PTL also follows all the expected semantics of normal Python code: so templates can have parameters, and the parameters can have default values, be treated as keyword arguments, etc.

The difference between a template and a regular Python function is that inside a template the result of expressions are saved as the return value of that template. Look at the first part of the example again:

template foo (x, y = 5):
    "This is a chunk of static text."
    greeting = "hello world" # statement, no PTL output
    print 'Input values:', x, y
    z = x + y
    """You can plug in variables like x (%s)
in a variety of ways.""" % x

Calling this template with foo(1,2) results in the following string:

This is a chunk of static text.You can plug in variables like x (1)
in a variety of ways.

Normally when Python evaluates expressions inside functions, it just discards their values, but in PTL the value is converted to a string using str() and appended to the template's return value. There's a single exception to this rule: None is the only value that's ever ignored, adding nothing to the output. (If this weren't the case, calling methods or functions that return None would require assigning their value to a variable. You'd have to write dummy = list.sort() in PTL code, which would be strange and confusing.)

The initial string in a template isn't treated as a docstring, but is just incorporated in the generated output; therefore, templates can't have docstrings. No whitespace is ever automatically added to the output, resulting in ...text.You can ... from the example. You'd have to add an extra space to one of the string literals to correct this.

The assignment to the greeting local variable is a statement, not an expression, so it doesn't return a value and produces no output. The output from the print statement will be printed as usual, but won't go into the string generated by the template. Quixote directs standard output into Quixote's debugging log; if you're using PTL on its own, you should consider doing something similar. print should never be used to generate output returned to the browser, only for adding debugging traces to a template.

Inside templates, you can use all of Python's control-flow statements:

template numbers(n):
    for i in range(n):
        " " # PTL does not add any whitespace

Calling numbers(5) will return the string "1 2 3 4 5 ". You can also have conditional logic or exception blocks:

template international_hello(language):
    if language == "english":
    elif language == "french":
        raise ValueError, "I don't speak %s" % language

PTL templates are kept in files with the extension .ptl. Like Python files, they are byte-compiled on import, and the byte-code is written to a compiled file with the extension .ptlc. Since vanilla Python doesn't know anything about PTL, Quixote provides an import hook to let you import PTL files just like regular Python modules. The standard way to install this import hook is by calling the enable_ptl() function:

from quixote import enable_ptl

(Note: if you're using ZODB, always import ZODB before installing the PTL import hook. There's some interaction which causes importing the TimeStamp module to fail when the PTL import hook is installed; we haven't debugged the problem.)

Once the import hook is installed, PTL files can be imported as if they were Python modules. If all the example templates shown here were put into a file named foo.ptl, you could then write Python code that did this:

from foo import numbers
def f():
    return numbers(10)

You may want to keep this little function in your PYTHONSTARTUP file:

def ptl():
        import ZODB
    except ImportError:
    from quixote import enable_ptl

This is useful if you want to interactively play with a PTL module.

$Id: PTL.txt,v 1.13 2002/10/02 14:52:47 gward Exp $