即使我通过了参数，也会在 kwargs.pop 中获取 KeyError答案

【问题标题】：Getting KeyError in kwargs.pop even though I pass the argument即使我通过了参数，也会在 kwargs.pop 中获取 KeyError
【发布时间】：2017-12-15 13:22:59
【问题描述】：

我有以下代码

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models

from .validators import audio_validator

# Create your models here.

from django.forms import forms
from django.template.defaultfilters import filesizeformat
from django.utils.translation import ugettext_lazy as _

MAX_FILE_SIZE=20971520

UPLOAD_TO = 'uploads'

class ContentTypeRestrictedFileField(models.FileField):

    def __init__(self, *args, **kwargs):
        self.content_types = kwargs.pop("content_types")
        self.max_upload_size = kwargs.pop("max_upload_size")

        super(ContentTypeRestrictedFileField, self).__init__(*args, **kwargs)

    def clean(self, *args, **kwargs):
        data = super(ContentTypeRestrictedFileField, self).clean(*args, **kwargs)

        file = data.file
        try:
            content_type = file.content_type
            if content_type in self.content_types:
                if file._size > self.max_upload_size:
                    raise forms.ValidationError(_('Please keep filesize under %s. Current filesize %s') % (filesizeformat(self.max_upload_size), filesizeformat(file._size)))
            else:
                raise forms.ValidationError(_('Filetype not supported.'))
        except AttributeError:
            pass

        return data

class AudioFile(models.Model):
    file_name = models.CharField(max_length=100)
    audio_file = ContentTypeRestrictedFileField(upload_to=UPLOAD_TO, validators=[audio_validator], content_types=['audio/aac', 'audio/mpeg', 'audio/ogg', 'audio/x-wav', 'audio/webm', 'audio/3gpp',], max_upload_size=MAX_FILE_SIZE)
    uploaded_at = models.DateTimeField(auto_now_add=True)

如您所见，我将content_types 作为列表传递，但它不起作用。当我运行`./manage.py migrate 时，我得到了

raceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/home/student/PycharmProjects/nightenv/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "/home/student/PycharmProjects/nightenv/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/student/PycharmProjects/nightenv/local/lib/python2.7/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/student/PycharmProjects/nightenv/local/lib/python2.7/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "/home/student/PycharmProjects/nightenv/local/lib/python2.7/site-packages/django/core/management/commands/migrate.py", line 184, in handle
    ProjectState.from_apps(apps),
  File "/home/student/PycharmProjects/nightenv/local/lib/python2.7/site-packages/django/db/migrations/state.py", line 230, in from_apps
    model_state = ModelState.from_model(model)
  File "/home/student/PycharmProjects/nightenv/local/lib/python2.7/site-packages/django/db/migrations/state.py", line 432, in from_model
    fields.append((name, field.clone()))
  File "/home/student/PycharmProjects/nightenv/local/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 469, in clone
    return self.__class__(*args, **kwargs)
  File "/home/student/PycharmProjects/tonightcore/nightcore/models.py", line 34, in __init__
    self.content_types = kwargs.pop("content_types")
KeyError: u'content_types'

如果我print(kwargs) 我明白了：

{'content_types': [u'audio/aac', u'audio/mpeg', u'audio/ogg', u'audio/x-wav', u'audio/webm', u'audio/3gpp'], 'max_upload_size': 20971520, 'upload_to': u'uploads', 'validators': [<django.core.validators.FileExtensionValidator object at 0x7fb90952a9d0>]}
{'upload_to': u'uploads', 'validators': [<django.core.validators.FileExtensionValidator object at 0x7fb909441090>]}

在异常之前。

【问题讨论】：

如果你在__init__方法的第一行打印kwargs，你会看到什么？
我认为 AudioFile 可能不会引发异常，可能是 django 本身的一些 init 进程。
@Harpal 已加入回答
@Yu-LinChen 已加入回答

标签： python keyerror keyword-argument

【解决方案1】：

阅读文档以了解 pop 的工作原理：

In [65]: dict.pop?
Docstring:
D.pop(k[,d]) -> v, remove specified key and return the corresponding value.
If key is not found, d is returned if given, otherwise KeyError is raised
Type:      method_descriptor

现在检查您的堆栈跟踪。看起来 django 正在尝试以某种方式实例化对象而不传递必要的关键字参数。所以最好给pop添加一个默认值

【讨论】：

【解决方案2】：

您可以像这样轻松地将默认参数添加到 self_variables（self.content_types 和 self.max_upload_size）...

def __init__(self, *args, **kwargs):
    self.content_types = kwargs.pop("content_types", [])
    self.max_upload_size = kwargs.pop("max_upload_size", [])

【讨论】：

OR 是什么意思？我很困惑
@LonelyNeuron 看起来像是另一个答案的替代品
@Lonely Neuron 做得很好。

【解决方案3】：

似乎 Django 实际上是在调用 ContentTypeRestrictedFileField 而没有我出于某种原因通过的 kwargs。

如果我按照某些用户的建议print(kwargs)，我发现这些自定义参数不存在。

也许 Django 对所有 ModelFiled 都这样做。

我向kwargs.pop 添加了一个默认参数，它现在可以工作了。甚至文件验证也有效。

我什至通过删除 Django 的默认文件扩展名验证器进行了测试。

def __init__(self, *args, **kwargs):
    print(kwargs)
    self.content_types = kwargs.pop("content_types", [])
    self.max_upload_size = kwargs.pop("max_upload_size", 5242880)

【讨论】：