Programmers usually want to use variables in their program to create different results dynamically from the same program. e.g. The first program for a newbie is usually
1 | print("Hello World") |
The second program will teach us how to get an input from keyboard and print it out.
1 | myinput = # get input from keyboard |
In a template engine, it does similar but much more powful thing. Usually, we call it a template language since it gives simple program capabilities, such as variables, loop and conditions.
Jinja2 is a template engine in python to combine a model(object values), expressions and statement tags with a template.
To understand what Jinja2 does, there are a few thing we need to understand firstly.
- Template, a predefined the file or string aligned to Jinja2 syntax or senmantic
- Model or variable, the variable used to replace the placeholder in your template
- Expression, a function used to change value of the variable
- Statement tags, logic control statements such as loop or if condition
- Commnent, description about a template which will be ignored by the engine when rendering
- Render, the procedure to replace variable or model in a template while following the statement controls
Initial look
Here is a simple tempalte from Jinja2
1 | <!DOCTYPE html> |
Let’s look at how jinja will render this template out when given variables. It’s better than just read the explanations.
1 | navigation = [{href='/', caption='Home'}] |
Delimiters
Delimiters is used by jinja engine to look up variables, expressions, statements and comments in a Template, Jinja runs different renders logic depending on the delimiters.
In a programmming language, the compiler or interpreter runs codes line by line, and understands our commands by keywords.
In Jinja2, it’s those special delimiters that the engine understands intially, and then the engine will try to understand the words between those delimiters.
There are a few kinds of delimiters. The default Jinja delimiters are configured as follows.
1 | {{ ... }} for Expressions and variables to print to the template output |
- The advanced users can choose different delimiters when using Jinja2
Expressions
Math
1 | {{ XX + - * / // % ** YY }} |
String plus
We can concatenate string
1 | {{ 'name' ~ name }} |
Comparisons
1 | {{ XX == != >= <= < > YY }} |
Logic
1 | {{ XX and or YY }} |
Test
A test expression is used against the is operator, a test expression is a function such as defined(value).
Full list of tests expression
1 | {{ XX in [YY,ZZ]}} |
When using a test expression in jinja template, we can ignore the only value parameter as following
1 | {{ XX is defined }} |
Some expressions have extra parameters, e.g. divisibleby(value, num)
1 | {{ XX is divisibleby(3) }} |
Object
We can use python method or attribute or items in a var object
1 | {{ XX.method() XX.YY XX[YY] }} |
Filter
We can alter a value using filter, or I think mapper is more suitable name, it’s similar as XX.map(mapper1).map(mapper2). Full list of filters
1 | {{ XX | filter1 | fitler2 }} |
Statements
Control statement
In a template, we can use for, if to control simple logic.
1 | <ul> |
https://jinja.palletsprojects.com/en/2.11.x/templates/#list-of-control-structures
Define macros
A macro is used to defined command used piece of codes and used in the template as a function.
1 | {% macro input(name, value='', type='text', size=20) -%} |
https://jinja.palletsprojects.com/en/2.11.x/templates/#macros
https://jinja.palletsprojects.com/en/2.11.x/templates/#call
Filters
There are built-in filters in Jinja2, such as upper, it can used by statements to make a block of contents to be uppcase
1 | {% filter upper %} |
Assignment
Some times we can to setup some variables in the template, the assginment statement fits in
1 | {% set navigation = [('index.html', 'Index'), ('about.html', 'About')] %} |
Block Assignments
Similar to assignment, we can capture the context in a block to a variable
1 | {% set navigation %} |
