Python 意外的导入行为答案

【问题标题】：Python unexpected import behaviourPython 意外的导入行为
【发布时间】：2016-02-18 22:21:24
【问题描述】：

我正在尝试构建一个视图，其中基于表单 slug 动态加载表单，可用表单在这样的元组列表中定义，这是为了加快“类似框架”中新表单的开发"方式：

#installed_io.py
forms = [("json",JsonForm),("csv",CsvForm),....]

froms 像往常一样在 forms.py 模块中定义。

from django import forms

class FileForm(BaseDatasetForm):
    file = forms.FileField(label="Opcion 1: Seleccione un archivo", required=False)
    text = forms.CharField(widget=forms.Textarea, label="Opction 2: Introduzca el contenido en este campo", required=False)

utils.py 定义了动态选择Form类的函数：

from installed_io import installed_inputs    

def get_input_form(slug):
    for entry in installed_inputs:
        if entry[0] == slug:
            return entry[1]
    raise NotImplementedError("The required form is not implemented or missing from the installed inputs")

视图是在我的 django 应用的 views.py 模块中定义的：

#views.py
from utils import get_input_form

@login_required
def add(request, slug):
    InputForm = get_input_form(slug)
    if request.method == "POST":
        form = InputForm(request.POST, request.FILES)
        if form.is_valid():
            object_id = form.save()
            messages.success(request, "Dataset created")
            return redirect(reverse("input:dataset", args=[str(object_id.inserted_id)]))
    else:
        form = InputForm()
    return render(request, "datasets/add-form.html", {"form":form})

但我收到此导入错误：

python manage.py runserver
Unhandled exception in thread started by <function wrapper at 0x7fe9c465c398>
Traceback (most recent call last):
  File "/home/jesus/workspace/tensorflow-board-django/venv/local/lib/python2.7/site-packages/django/utils/autoreload.py", line 226, in wrapper
    fn(*args, **kwargs)
  File "/home/jesus/workspace/tensorflow-board-django/venv/local/lib/python2.7/site-packages/django/core/management/commands/runserver.py", line 109, in inner_run
    autoreload.raise_last_exception()
  File "/home/jesus/workspace/tensorflow-board-django/venv/local/lib/python2.7/site-packages/django/utils/autoreload.py", line 249, in raise_last_exception
    six.reraise(*_exception)
  File "/home/jesus/workspace/tensorflow-board-django/venv/local/lib/python2.7/site-packages/django/utils/autoreload.py", line 226, in wrapper
    fn(*args, **kwargs)
  File "/home/jesus/workspace/tensorflow-board-django/venv/local/lib/python2.7/site-packages/django/__init__.py", line 18, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/home/jesus/workspace/tensorflow-board-django/venv/local/lib/python2.7/site-packages/django/apps/registry.py", line 108, in populate
    app_config.import_models(all_models)
  File "/home/jesus/workspace/tensorflow-board-django/venv/local/lib/python2.7/site-packages/django/apps/config.py", line 202, in import_models
    self.models_module = import_module(models_module_name)
  File "/usr/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/home/jesus/workspace/tensorflow-board-django/tensorflow_board_django/analysis/models.py", line 2, in <module>
    from ..input.forms import SOURCES
  File "/home/jesus/workspace/tensorflow-board-django/tensorflow_board_django/input/forms.py", line 2, in <module>
    from utils import save_dataset
  File "/home/jesus/workspace/tensorflow-board-django/tensorflow_board_django/input/utils.py", line 2, in <module>
    from installed_io import installed_inputs
  File "/home/jesus/workspace/tensorflow-board-django/tensorflow_board_django/input/installed_io.py", line 1, in <module>
    from forms import FileForm
ImportError: cannot import name FileForm

我尝试过的：

使用绝对路径更改导入语句
删除 pyc 文件
使用点符号导入模块
尝试将 installed_input 列表保存在 init.py
只有将所有代码移动到 views.py 文件才有效，但我发现此解决方案非常单一且非 Python 化。

【问题讨论】：

我在您的堆栈跟踪中看到循环导入，转到 forms -> utils -> installed_io -> forms。这些会导致各种初始化顺序问题，可能包括您在此处看到的问题。尽可能避免使用它们。
那是circular import 的味道，或者是 FileForm 本身的错误。尝试运行 shell ./manage.py shell 和 from .. import FileForm 以确保它们独立工作。然后在你的 shell 中尝试from utils import get_input_form。
manage.py shell 会抛出同样的异常，将 utils.py 和 installed_io.py 合并到 views.py 确实解决了导入问题。
您可以在闭包中（在函数调用中）进行输入以进行测试，但我建议您重新编写导入以避免从长远来看任何循环导入。
如果您愿意，请以解决这种循环依赖的最佳方式发布对此答案的回复，否则我将在稍后发布我的解决方案。

标签： python django python-2.7 python-import

【解决方案1】：

基于原始帖子中的 cmets，我对代码进行了以下更改：

我没有在 installed_io 模块上导入类，而是使用字符串：

#installed_io.py
forms = [("json","JsonForm"),("csv","CsvForm"),....]

然后我更改了 utils.py 以从字符串导入类：

from installed_io import installed_inputs    

def get_input_form(slug):
    for entry in installed_inputs:
        if entry[0] == slug:
            module = importlib.import_module("tensorflow_board_django.io.forms")
            class_name = entry[1]
            return getattr(module, class_name)
    raise NotImplementedError("The required form is not implemented or missing from the installed inputs")

【讨论】：