运行出现这个错误

TypeError at /goods/

__str__ returned non-string (type NoneType)

Request Method:

GET    

Request URL:

http://127.0.0.1:8000/goods/    

Django Version:

1.11.3    

Exception Type:

TypeError    

Exception Value:

__str__ returned non-string (type NoneType)
   

Exception Location:

/Users/bill/demo/VueShop/lib/python3.6/site-packages/django/utils/functional.py in inner, line 239    

Python Executable:

/Users/bill/demo/VueShop/bin/python3.6    

Python Version:

3.6.3    

Python Path:

['/Users/bill/Documents/GitHub/MxShop/extra_apps',
'/Users/bill/Documents/GitHub/MxShop/apps',
'/Users/bill/Documents/GitHub/MxShop',
'/Users/bill/Documents/GitHub/MxShop',
'/Applications/PyCharm.app/Contents/helpers/pydev',
'/Users/bill/Documents/GitHub/MxShop',
'/Users/bill/Documents/GitHub/MxShop/apps',
'/Users/bill/Documents/GitHub/MxShop/extra_apps',
'/Applications/PyCharm.app/Contents/helpers/pydev',
'/Users/bill/Library/Caches/PyCharm2017.3/cythonExtensions',
'/Users/bill/demo/VueShop/lib/python36.zip',
'/Users/bill/demo/VueShop/lib/python3.6',
'/Users/bill/demo/VueShop/lib/python3.6/lib-dynload',
'/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6',
'/Users/bill/demo/VueShop/lib/python3.6/site-packages',
'/Applications/PyCharm.app/Contents/helpers/pycharm_matplotlib_backend']
   

Server time:

星期三, 2 五月 2018 15:52:38 +0800    

Error during template rendering

In template /Users/bill/demo/VueShop/lib/python3.6/site-packages/rest_framework/templates/rest_framework/base.html, error at line 49

__str__ returned non-string (type NoneType)

39

<span>    

40

{% block branding %}    

41

<a class='navbar-brand' rel="nofollow" href='http://www.django-rest-framework.org'>    

42

Django REST framework    

43

</a>    

44

{% endblock %}    

45

</span>    

46

<ul class="nav navbar-nav pull-right">    

47

{% block userlinks %}    

48

{% if user.is_authenticated %}    

49

{% optional_logout request user %}    

50

{% else %}    

51

{% optional_login request %}    

52

{% endif %}    

53

{% endblock %}    

54

</ul>    

55

</div>    

56

</div>    

57

{% endblock %}    

58

59

<div class="container">    

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