即使在venv / virtualenv中，我也无法导入flask。随时编辑我的问题，使其更易于理解。
Flask==1.1.1
Flask-SQLAlchemy==2.4.0
Flask-WTF==0.14.2
sixie6e@--------:~$ python3
Python 3.7.4 (default, Sep 1 2019, 14:41:16)
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from flask import Flask
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'flask'
>>> exit()
sixie6e@---------:~$ pip3 install Flask
Collecting Flask
Using cached files.pythonhosted.org/packages/9b/93/628509b8d5dc749656a9641f4caf13540e2cdec85276964ff8f43bbb1d3b/Flask-1.1.1-py2.py3-none-any.whl
Collecting Jinja2>=2.10.1 (from Flask)
Using cached files.pythonhosted.org/packages/1d/e7/fd8b501e7a6dfe492a433deb7b9d833d39ca74916fa8bc63dd1a4947a671/Jinja2-2.10.1-py2.py3-none-any.whl
Collecting Werkzeug>=0.15 (from Flask)
Using cached files.pythonhosted.org/packages/d1/ab/d3bed6b92042622d24decc7aadc8877badf18aeca1571045840ad4956d3f/Werkzeug-0.15.5-py2.py3-none-any.whl
Collecting itsdangerous>=0.24 (from Flask)
Using cached files.pythonhosted.org/packages/76/ae/44b03b253d6fade317f32c24d100b3b35c2239807046a4c953c7b89fa49e/itsdangerous-1.1.0-py2.py3-none-any.whl
Collecting click>=5.1 (from Flask)
Using cached files.pythonhosted.org/packages/fa/37/45185cb5abbc30d7257104c434fe0b07e5a195a6847506c074527aa599ec/Click-7.0-py2.py3-none-any.whl
Collecting MarkupSafe>=0.23 (from Jinja2>=2.10.1->Flask)
Using cached files.pythonhosted.org/packages/6e/57/d40124076756c19ff2269678de7ae25a14ebbb3f6314eb5ce9477f191350/MarkupSafe-1.1.1-cp35-cp35m-manylinux1_x86_64.whl
Installing collected packages: MarkupSafe, Jinja2, Werkzeug, itsdangerous, click, Flask
Successfully installed Flask-1.1.1 Jinja2-2.10.1 MarkupSafe-1.1.1 Werkzeug-0.15.5 click-7.0 itsdangerous-1.1.0
sixie6e@-----------:~$ pip3 install flask
Collecting flask
Using cached files.pythonhosted.org/packages/9b/93/628509b8d5dc749656a9641f4caf13540e2cdec85276964ff8f43bbb1d3b/Flask-1.1.1-py2.py3-none-any.whl
Collecting Werkzeug>=0.15 (from flask)
Using cached files.pythonhosted.org/packages/d1/ab/d3bed6b92042622d24decc7aadc8877badf18aeca1571045840ad4956d3f/Werkzeug-0.15.5-py2.py3-none-any.whl
Collecting click>=5.1 (from flask)
Using cached files.pythonhosted.org/packages/fa/37/45185cb5abbc30d7257104c434fe0b07e5a195a6847506c074527aa599ec/Click-7.0-py2.py3-none-any.whl
Collecting Jinja2>=2.10.1 (from flask)
Using cached files.pythonhosted.org/packages/1d/e7/fd8b501e7a6dfe492a433deb7b9d833d39ca74916fa8bc63dd1a4947a671/Jinja2-2.10.1-py2.py3-none-any.whl
Collecting itsdangerous>=0.24 (from flask)
Using cached files.pythonhosted.org/packages/76/ae/44b03b253d6fade317f32c24d100b3b35c2239807046a4c953c7b89fa49e/itsdangerous-1.1.0-py2.py3-none-any.whl
Collecting MarkupSafe>=0.23 (from Jinja2>=2.10.1->flask)
Using cached files.pythonhosted.org/packages/6e/57/d40124076756c19ff2269678de7ae25a14ebbb3f6314eb5ce9477f191350/MarkupSafe-1.1.1-cp35-cp35m-manylinux1_x86_64.whl
Installing collected packages: Werkzeug, click, MarkupSafe, Jinja2, itsdangerous, flask
Successfully installed Jinja2-2.10.1 MarkupSafe-1.1.1 Werkzeug-0.15.5 click-7.0 flask-1.1.1 itsdangerous-1.1.0
sixie6e@----------:~$ python3 -m flask --version
/usr/local/bin/python3: No module named flask
sixie6e@----------:~$ python3 -m Flask --version
/usr/local/bin/python3: No module named Flask
(virtualenvironment) sixie6e@----------:~$ pip3 install Flask
Collecting Flask
Using cached files.pythonhosted.org/packages/9b/93/628509b8d5dc749656a9641f4caf13540e2cdec85276964ff8f43bbb1d3b/Flask-1.1.1-py2.py3-none-any.whl
Collecting itsdangerous>=0.24 (from Flask)
Using cached files.pythonhosted.org/packages/76/ae/44b03b253d6fade317f32c24d100b3b35c2239807046a4c953c7b89fa49e/itsdangerous-1.1.0-py2.py3-none-any.whl
Collecting Jinja2>=2.10.1 (from Flask)
Using cached files.pythonhosted.org/packages/1d/e7/fd8b501e7a6dfe492a433deb7b9d833d39ca74916fa8bc63dd1a4947a671/Jinja2-2.10.1-py2.py3-none-any.whl
Collecting Werkzeug>=0.15 (from Flask)
Using cached files.pythonhosted.org/packages/d1/ab/d3bed6b92042622d24decc7aadc8877badf18aeca1571045840ad4956d3f/Werkzeug-0.15.5-py2.py3-none-any.whl
Collecting click>=5.1 (from Flask)
Using cached files.pythonhosted.org/packages/fa/37/45185cb5abbc30d7257104c434fe0b07e5a195a6847506c074527aa599ec/Click-7.0-py2.py3-none-any.whl
Collecting MarkupSafe>=0.23 (from Jinja2>=2.10.1->Flask)
Using cached files.pythonhosted.org/packages/6e/57/d40124076756c19ff2269678de7ae25a14ebbb3f6314eb5ce9477f191350/MarkupSafe-1.1.1-cp35-cp35m-manylinux1_x86_64.whl
Installing collected packages: itsdangerous, MarkupSafe, Jinja2, Werkzeug, click, Flask
Successfully installed Flask-1.1.1 Jinja2-2.10.1 MarkupSafe-1.1.1 Werkzeug-0.15.5 click-7.0 itsdangerous-1.1.0
(virtualenvironment) sixie6e@-------------:~$ python3
Python 3.7.4 (default, Sep 1 2019, 14:41:16)
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import flask
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'flask'
>>> import Flask
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'Flask'
>>>