与virtualenvs和Python软件包的混淆

在我的python程序(使用python3.5由virtualenv运行)中，我需要使用Pillow库来处理图像.

In my python program (run by a virtualenv using python3.5), I need to use the Pillow library to process an image.

ImportError:没有名为枕头"的模块

ImportError: No module named 'Pillow'

告诉我在virtualenv中未安装枕头.

tells me that Pillow is not installed in the virtualenv.

但是，当我运行pip install Pillow时，我会回来:

But, when I run pip install Pillow, I get back:

已经满足要求:放在/usr/lib/python3/dist-packages中

Requirement already satisfied: Pillow in /usr/lib/python3/dist-packages

如果我正在使用的pip来自virtualenv，那么为什么要在/usr/lib/python3/dist-packages中查找以检查软件包是否已安装?

If the pip I am using is from the virtualenv, then why is it looking in /usr/lib/python3/dist-packages to check if the package is already installed?

仅需确保，我运行type python和type pip来确认这两个程序来自我的virtualenv，它们分别是:

Just to make sure, I run type python and type pip to confirm that these 2 programs are from my virtualenv, and they are:

python被散列了(/home/nelson/.virtualenvs/MainEnv/bin/python)

python is hashed (/home/nelson/.virtualenvs/MainEnv/bin/python)

pip进行了哈希处理(/home/nelson/.virtualenvs/MainEnv/bin/pip)

pip is hashed (/home/nelson/.virtualenvs/MainEnv/bin/pip)

创建virtualenv时不使用

sudo(我知道，因为这已经给我造成了问题)或尝试pip install时，未使用

sudo；那么，这种逻辑的缺陷在哪里呢?如何在我的virtualenv中安装枕头/如何导入枕头?

sudo was not used when creating the virtualenv (I know because this had already caused problems for me) or when trying to pip install; so where is the flaw in this logic? How can I install Pillow in my virtualenv / How can I import Pillow?