TopolearnFilters are how templates format data without Python code, and tests make conditions readable.
A filter transforms a value with the pipe syntax: {{ name|title }}. Filters chain left to right and can take arguments: {{ items|sort(attribute="price")|join(", ") }}.
Built-in filters you will use constantly:
- text:
upper,lower,title,capitalize,trim,truncate(30),replace("a", "b"),wordwrap,striptags - fallback:
default("n/a")(aliasd);default(x, true)also replaces empty or falsy values - collections:
length,first,last,join,sort,unique,reverse,sum(attribute="price"),batch(3),groupby("category"),dictsort,items - functional style:
map(attribute="name"),selectattr("active"),rejectattr("draft"),select("odd") - numbers and output:
round(2),int,float,filesizeformat,format,tojson,e,safe
map, select and friends return generators; finish with join, list or first.
A test checks a value with is: {% if user is defined %}, {% if n is divisibleby(3) %}, {% if x is none %}, is string, is number, is even, is iterable, is sameas(false). Negate with is not. Tests also power select("odd") and selectattr("age", "ge", 18).
Filters and tests are applied before most operators, so {{ -3|abs }} is -(3|abs); use parentheses: {{ (-3)|abs }}.
Example
from jinja2 import Environment
env = Environment()
t = env.from_string("""{{ title|default("Untitled")|upper }}
{{ products|selectattr("in_stock")|map(attribute="name")|join(", ") }}
Total: {{ products|sum(attribute="price")|round(2) }}
{% for g in products|groupby("cat") %}{{ g.grouper }}={{ g.list|length }} {% endfor %}
{% for n in range(1, 7) if n is divisibleby(3) %}{{ n }} {% endfor %}
{{ bio|default("-", true) }} {{ note is defined }}""")
print(t.render(bio="", products=[
{"name": "Pen", "price": 1.25, "cat": "office", "in_stock": True},
{"name": "Ink", "price": 4.5, "cat": "office", "in_stock": False},
{"name": "Mug", "price": 7.333, "cat": "kitchen", "in_stock": True}]))
# UNTITLED
# Pen, Mug
# Total: 13.08
# kitchen=1 office=2
# 3 6
# - FalseRemember
- value|filter(args) transforms; filters chain left to right
- default, join, sort, map, selectattr and groupby cover most formatting
- Tests use is: is defined, is none, is divisibleby(n)
Watch out for
- Forgetting that map/select return generators; finish with join, list or first.