Compiling Templates - Python

Compiling a template converts it into Python code, which eliminates the need for Cheetah to go through a template and replace everything. This means that Cheetah can work faster. Compiling a template is very simple, and all it involves is a call to Cheetah. Take a look at this template, quote.tmpl, which provides a placeholder for a random quotation:

<center>
<i>$quotation</i><br />
-- $speaker
</center>

To compile this, we pass a few arguments to cheetah:

cheetah compile quote.tmpl

Cheetah then generates the file quote.py, which contains the class quote. The class can be imported into a Python script and then used similarly to the Template class:

import random
from quote import quote

# Define a list of quotations
quotations = [ [ 'It is easier to find people fit to govern
themselves than people fit to govern others.', 'Lord Acton' ],\
               [ 'A house divided against itself cannot stand.',
'Abraham Lincoln' ],\
               [ 'Before anything else, preparation is the key to
success.', 'Alexander Graham Bell' ] ]

# Pick a random quotation
quotation = random.choice ( quotations )

# Print the product
print quotation ( searchList = [{ 'quotation': quotation [0],
'author': quotation [1] }]

Adding Logic

Instead of having a Python script control all the logic for a certain template, it's possible to slip a bit of simple logic into a template. Cheetah makes this possible through special directives. For example, say that you want to display a certain message if one condition is met and a certain message if another condition is met. The #if directive can be used for this task, rather than doing work in Python. Take a look at greeting.tmpl, which displays different greetings for different times of day:

#if $hour < 12
Good morning!
#else if $hour >= 12 and $hour <=18
Good afternoon!
#else
Good evening!
#end if

All we need to do is pass the hour of the day to the script, and it will display the appropriate message:

import time
from Cheetah.Template import Template

hour = time.localtime() [3]
print Template ( file = 'greeting.tmpl', searchList = [{ 'hour':
hour }] )

The #unless directive is similar to the #if directive, but it returns the opposite of what the expression returns. If the expression is true, then the contained code is not executed. If the expression is false, then the contained code executes. Take this template for example:

#unless $a
< Message >
#end unless

Unless $a returns a true value, “< Message >” is displayed to the user. If $a does return a true value, then the message is never displayed.

Cheetah also allows for loops to be used within templates. The #for directive acts as a for loop, as in listPresidents.tmpl:

Presidents:
#for $name in $names:
<br />$name
#end for

To see the template in action, we need to pass a list to take the place of the $names variable:

from Cheetah.Template import Template

presidents = [ 'George Washington', 'John Adams', 'Thomas
Jefferson', 'James Madison', 'James Monroe' ]

print Template ( file = 'listPresidents.tmpl', searchList =
[{ 'names': presidents }] )

The #repeat directive simply repeats something a specified amount of times. For example, say I wanted to make a primitive horizontal bar graph using “|” characters. I could use the #repeat directive to do this in graph.tmpl:

#repeat $x
|
#end repeat

We can now substitute $x for the length of the graph:

from Cheetah.Template import Template

# Set the length of the bar graph
x = 10

print Template ( file = 'graph.tmpl', searchList = [{ 'x': x }] )

If you run the above script, however, you will see that each “|” character is put on a new line. This can be fixed using the #slurp directive:

#repeat $x
|#slurp
#end repeat

Lastly, the #while directive acts as a while loop:

#set $x = 1
#while $x <= 10
$x
#set $x = $x + 1
#end while

Notice that the above template uses the #set directive. This simply sets the value of a variable.

Cheetah also contains #break and #continue directives for use in loops. This template will not display “4” or anything past “5”:

#set $x = 1
#while $x <= 10
#if $x == 4
#continue
#end if
$x
#if $x ==5
#break
#set $x = $x + 1
#end if
#end while