Python packages hash not matching whilst installing using pip

I am using pip to install all my python packages but get error as shown in the trace below. What is the problem and how can I solve it?

usr@comp:~$ pip install flask
    Collecting flask
      Using cached Flask-0.11.1-py2.py3-none-any.whl
    Collecting itsdangerous>=0.21 (from flask)
      Using cached itsdangerous-0.24.tar.gz
    Collecting click>=2.0 (from flask)
      Using cached click-6.6.tar.gz
    Collecting Werkzeug>=0.7 (from flask)
      Using cached Werkzeug-0.11.11-py2.py3-none-any.whl
    Requirement already satisfied (use --upgrade to upgrade): Jinja2>=2.4 in /usr/lib/python2.7/dist-packages (from flask)
    Requirement already satisfied (use --upgrade to upgrade): MarkupSafe in /usr/lib/python2.7/dist-packages (from Jinja2>=2.4->flask)
    THESE PACKAGES DO NOT MATCH THE HASHES FROM THE REQUIREMENTS FILE. If you have updated the package versions, please update the hashes. Otherwise, examine the package contents carefully; someone may have tampered with them.
        Werkzeug>=0.7 from https://pypi.python.org/packages/a9/5e/41f791a3f380ec50f2c4c3ef1399d9ffce6b4fe9a7f305222f014cf4fe83/Werkzeug-0.11.11-py2.py3-none-any.whl#md5=c63a21eedce9504d223ed89358c4bdc9 (from flask):
        Expected md5 c63a21eedce9504d223ed89358c4bdc9
             Got        13a168aafcc43354b6c79ef44bb0dc71

There is a similar problem (Why does pip fail with bad md5 hash for package?) from 2013 the solution that I tried that worked for me is this:

sudo pip install --no-cache-dir flask

given by attolee

--no-cache-dir did not work for me in raspberry pi 4 at first.

Found that the problem was due to unexpected network change/failure during pip installation

I had to download the broken .whl file manually with wget

and install it like below: sudo pip install scipy-1.3.0-cp37-cp37m-linux_armv7l.whl

followed by sudo pip install --no-cache-dir keras

Then it worked.