django的模板编译后变成一般text、tag、varible，然后根据上下文进行渲染。
class ForNode(Node):
child_nodelists = ('nodelist_loop', 'nodelist_empty')
def __init__(self, loopvars, sequence, is_reversed, nodelist_loop, nodelist_empty=None):
self.loopvars, self.sequence = loopvars, sequence
self.is_reversed = is_reversed
self.nodelist_loop = nodelist_loop
if nodelist_empty is None:
self.nodelist_empty = NodeList()
else:
self.nodelist_empty = nodelist_empty
def __repr__(self):
reversed_text = ' reversed' if self.is_reversed else ''
return "<For Node: for %s in %s, tail_len: %d%s>" % \
(', '.join(self.loopvars), self.sequence, len(self.nodelist_loop),
reversed_text)
def __iter__(self):
for node in self.nodelist_loop:
yield node
for node in self.nodelist_empty:
yield node
def render(self, context):
if 'forloop' in context:#先判断是不是for循环
parentloop = context['forloop']
else:
parentloop = {}
with context.push():#调用context.push()会在栈顶放一个contextdict对象
try:
values = self.sequence.resolve(context, True)#找到可迭代的对象
except VariableDoesNotExist:
values = []
if values is None:
values = []
if not hasattr(values, '__len__'):
values = list(values)
len_values = len(values)
if len_values < 1:
return self.nodelist_empty.render(context)
nodelist = []
if self.is_reversed:
values = reversed(values)
num_loopvars = len(self.loopvars)#判断loop变量个数
unpack = num_loopvars > 1
# Create a forloop value in the context. We'll update counters on each
# iteration just below.
loop_dict = context['forloop'] = {'parentloop': parentloop}#loop_dict与context['forloop']是同一个词典，loop_dict['counter0'] = i后，通过context['forloop']可以访问到loop_dict
for i, item in enumerate(values):
# Shortcuts for current loop iteration number.
loop_dict['counter0'] = i#每次循环时counter0的值都会变化，可以在模板文件上使用
loop_dict['counter'] = i + 1
# Reverse counter iteration numbers.
loop_dict['revcounter'] = len_values - i
loop_dict['revcounter0'] = len_values - i - 1
# Boolean values designating first and last times through loop.
loop_dict['first'] = (i == 0)
loop_dict['last'] = (i == len_values - 1)
pop_context = False
if unpack:
# If there are multiple loop variables, unpack the item into
# them.
# To complete this deprecation, remove from here to the
# try/except block as well as the try/except itself,
# leaving `unpacked_vars = ...` and the "else" statements.
if not isinstance(item, (list, tuple)):
len_item = 1
else:
len_item = len(item)
# Check loop variable count before unpacking
if num_loopvars != len_item:
warnings.warn(
"Need {} values to unpack in for loop; got {}. "
"This will raise an exception in Django 1.10."
.format(num_loopvars, len_item),
RemovedInDjango110Warning)
try:
unpacked_vars = dict(zip(self.loopvars, item))
except TypeError:
pass
else:
pop_context = True
context.update(unpacked_vars)
else:
context[self.loopvars[0]] = item#在上下文中增加变量，为后面的node渲染提供数据
for node in self.nodelist_loop:
nodelist.append(node.render_annotated(context))
if pop_context:
# The loop variables were pushed on to the context so pop them
# off again. This is necessary because the tag lets the length
# of loopvars differ to the length of each set of items and we
# don't want to leave any vars from the previous loop on the
# context.
context.pop()
return mark_safe(''.join(force_text(n) for n in nodelist))
context每次调用push函数后都会改变context的dicts列表。解析一个node时，都会有一个上下文。相当于一个新的环境。