pip install pipreqs

Obs.: if you don't want support for jupyter notebooks, you can install pipreqs without the dependencies that give support to it. To do so, run:

pip install --no-deps pipreqs
pip install yarg==0.1.9 docopt==0.6.2

Usage:
    pipreqs [options] [<path>]
Arguments:
    <path>                The path to the directory containing the application files for which a requirements file
                          should be generated (defaults to the current working directory)
Options:
    --use-local           Use ONLY local package info instead of querying PyPI
    --pypi-server <url>   Use custom PyPi server
    --proxy <url>         Use Proxy, parameter will be passed to requests library. You can also just set the
                          environments parameter in your terminal:
                          $ export HTTP_PROXY="https://10.10.1.10:3128"
                          $ export HTTPS_PROXY="https://10.10.1.10:1080"
    --debug               Print debug information
    --ignore <dirs>...    Ignore extra directories, each separated by a comma
    --no-follow-links     Do not follow symbolic links in the project
    --ignore-errors       Ignore errors while scanning files
    --encoding <charset>  Use encoding parameter for file open
    --savepath <file>     Save the list of requirements in the given file
    --print               Output the list of requirements in the standard output
    --force               Overwrite existing requirements.txt
    --diff <file>         Compare modules in requirements.txt to project imports
    --clean <file>        Clean up requirements.txt by removing modules that are not imported in project
    --mode <scheme>       Enables dynamic versioning with <compat>, <gt> or <non-pin> schemes
                          <compat> | e.g. Flask~=1.1.2
                          <gt>     | e.g. Flask>=1.1.2
                          <no-pin> | e.g. Flask
    --scan-notebooks      Look for imports in jupyter notebook files.

$ pipreqs /home/project/location
Successfully saved requirements file in /home/project/location/requirements.txt

Contents of requirements.txt

wheel==0.23.0
Yarg==0.1.9
docopt==0.6.2

• pip freeze only saves the packages that are installed with pip install in your environment.
• pip freeze saves all packages in the environment including those that you don't use in your current project (if you don't have virtualenv).
• and sometimes you just need to create requirements.txt for a new project without installing modules.