flask，jinja2：jinja2.exceptions.UndefinedError：'ngettext'未定义

Question

我正在编写一个小烧瓶应用程序。这是我的Python代码：

from flask import Flask, render_template
from jinja2 import Environment

from towngen import generate_settlement


app = Flask(__name__)
jinja_env = Environment(extensions=['jinja2.ext.i18n'])
app.jinja_options['extensions'].append('jinja2.ext.i18n')

@app.route("/")
def hello_world():
    return render_template("settlement.html", settlement=generate_settlement())

我使用的模板内部：

    {% trans count=settlement.features|length %}
    The settlement has {apnumber(count)} unique features:
    {% pluralize %}
    The settlement has {apnumber(count)} unique features:
    {% endtrans %}

这会引发异常：

Traceback (most recent call last):
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py", line 2464, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py", line 2450, in wsgi_app
    response = self.handle_exception(e)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py", line 1867, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/gru/Code/towngen/web.py", line 13, in hello_world
    return render_template("settlement.html", settlement=generate_settlement())
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/templating.py", line 137, in render_template
    return _render(
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/flask/templating.py", line 120, in _render
    rv = template.render(context)
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/jinja2/environment.py", line 1090, in render
    self.environment.handle_exception()
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/jinja2/environment.py", line 832, in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/Users/gru/.venvs/towngen/lib/python3.8/site-packages/jinja2/_compat.py", line 28, in reraise
    raise value.with_traceback(tb)
  File "/Users/gru/Code/towngen/templates/settlement.html", line 12, in top-level template code
    {% trans count=settlement.features|length %}
jinja2.exceptions.UndefinedError: 'ngettext' is undefined

我是否配置错误？我找不到可以清楚地描述如何添加该扩展的文档。

Answer 1

昨天我的 Jinja 3.1 项目也遇到了同样的问题，基于 那里的模板设计器文档。

我也没有看到任何明显的解释——如果还没有的话，我会考虑为此提交一张票——但事实证明，你和我在设置 i18n 扩展时都缺少一个额外的步骤。

来自扩展 API 文档的 i18n 部分：

启用后，应用程序必须在全局或渲染时提供

gettext

、

ngettext

以及可选的

pgettext

和

npgettext

的功能。

_()

函数作为别名添加到

gettext

函数。

如果您执行所有这些操作只是为了启用

{% pluralize %}

标记，我的建议是添加对您环境的

install_null_translations

方法的调用。这是我的做法，立即解决了问题：

from jinja2 import Environment, FileSystemLoader, select_autoescape

_env = Environment(
    loader=FileSystemLoader(_TEMPLATES_DIR),
    extensions=['jinja2.ext.i18n'],
    autoescape=select_autoescape(default=True, default_for_string=True))

# ADD THIS NEXT LINE ---------------------------------------------------
_env.install_null_translations()