title: Why does the pip requirements file contain "@file" instead of version number?
body: <p>I created the requirements.txt with pip freeze &gt; requirements.txt. Some modules show the @file..... instead of the version #. What does it mean and why it show?</p>
  <p>Conda: 4.8.3</p>
  <p>Here is the result of requirements.txt. e.g. astroid, flask-admin, matplotlib shows &quot;@ file&quot; below</p>
  <pre><code>astroid @ file:///opt/concourse/worker/volumes/live/b22b518b-f584-4586-5ee9-55bfa4fca96e/volume/astroid_1592495912194/work
  bcrypt==3.1.7
  blinker==1.4
  certifi==2020.6.20
  cffi==1.14.0
  click==7.1.2
  cycler==0.10.0
  dnspython==1.16.0
  ecdsa==0.13
  email-validator @ file:///home/conda/feedstock_root/build_artifacts/email_validator_1589962946737/work
  flake8==3.8.3
  Flask==1.1.2
  Flask-Admin @ file:///tmp/build/80754af9/flask-admin_1592429635880/work
  Flask-Bcrypt==0.7.1
  Flask-Login==0.5.0
  Flask-Mail==0.9.1
  flask-msearch==0.2.9
  Flask-SQLAlchemy==2.4.3
  Flask-WTF==0.14.3
  gunicorn==20.0.4
  idna==2.9
  importlib-metadata==1.6.1
  isort==4.3.21
  itsdangerous==1.1.0
  Jinja2==2.11.2
  kiwisolver==1.2.0
  lazy-object-proxy==1.4.3
  MarkupSafe==1.1.1
  matplotlib @ file:///Users/runner/miniforge3/conda-bld/matplotlib-base_1592576116805/work
  mccabe==0.6.1
  mkl-fft==1.1.0
  mkl-random==1.1.1
  mkl-service==2.3.0
  numpy==1.18.5
  pandas @ file:///opt/concourse/worker/volumes/live/38d1301c-8fa9-4d2f-662e-34dddf33b183/volume/pandas_1592841668171/work
  psycopg2==2.8.4
  pycodestyle @ file:///home/conda/feedstock_root/build_artifacts/pycodestyle_1589305246696/work
  pycparser==2.20
  pycryptodome==3.9.7
  pyflakes==2.2.0
  pylint @ file:///opt/concourse/worker/volumes/live/42ede439-2571-4cb2-513c-394625d2381b/volume/pylint_1592496039330/work
  pyparsing==2.4.7
  python-dateutil==2.8.1
  pytz==2020.1
  six @ file:///home/conda/feedstock_root/build_artifacts/six_1590081179328/work
  SQLAlchemy==1.3.17
  toml @ file:///tmp/build/80754af9/toml_1592853716807/work
  tornado==6.0.4
  typed-ast==1.4.1
  Werkzeug==1.0.1
  wrapt==1.11.2
  WTForms==2.3.1
  xlrd==1.2.0
  zipp==3.1.0
  </code></pre>
  <p>Here is the conda list</p>
  <pre><code>astroid                   2.4.2                    py37_0    anaconda
  bcrypt                    3.1.7            py37h9bfed18_1    conda-forge
  blas                      1.0                         mkl    anaconda
  blinker                   1.4                        py_1    conda-forge
  ca-certificates           2020.1.1                      0    anaconda
  certifi                   2020.6.20                py37_0    anaconda
  cffi                      1.14.0           py37h356ff06_0    conda-forge
  click                     7.1.2                      py_0    anaconda
  cycler                    0.10.0                     py_2    conda-forge
  dnspython                 1.16.0                     py_1    conda-forge
  ecdsa                     0.13                       py_0    conda-forge
  email_validator           1.1.1              pyh9f0ad1d_0    conda-forge
  flake8                    3.8.3                      py_0    anaconda
  flask                     1.1.2                      py_0    anaconda
  flask-admin               1.5.4                      py_0    anaconda
  flask-bcrypt              0.7.1                      py_1    conda-forge
  flask-login               0.5.0                      py_0    anaconda
  flask-mail                0.9.1                      py_2    conda-forge
  flask-msearch             0.2.9                    pypi_0    pypi
  flask-sqlalchemy          2.4.3                    pypi_0    pypi
  flask-wtf                 0.14.3                     py_0    anaconda
  freetype                  2.10.2               h8da9a1a_0    conda-forge
  gmp                       6.2.0                h4a8c4bd_2    conda-forge
  gunicorn                  20.0.4                   py37_0    anaconda
  idna                      2.9                        py_1    conda-forge
  importlib-metadata        1.6.1                    py37_0    anaconda
  intel-openmp              2020.1                      216    anaconda
  isort                     4.3.21                   py37_0    anaconda
  itsdangerous              1.1.0                    py37_0    anaconda
  jinja2                    2.11.2                     py_0    anaconda
  kiwisolver                1.2.0            py37ha1cc60f_0    conda-forge
  krb5                      1.16.4               hddcf347_0    anaconda
  lazy-object-proxy         1.4.3            py37h1de35cc_0    anaconda
  libcxx                    10.0.0                        1  
  libedit                   3.1.20191231         haf1e3a3_0  
  libffi                    3.2.1                h0a44026_6  
  libgfortran               3.0.1                h93005f0_2    anaconda
  libpng                    1.6.37               hbbe82c9_1    conda-forge
  libpq                     11.2                 h051b688_0    anaconda
  markupsafe                1.1.1            py37h1de35cc_0    anaconda
  matplotlib                3.2.2                         0    conda-forge
  matplotlib-base           3.2.2            py37hddda452_0    conda-forge
  mccabe                    0.6.1                    py37_1    anaconda
  mkl                       2019.4                      233    anaconda
  mkl-service               2.3.0            py37hfbe908c_0    anaconda
  mkl_fft                   1.1.0            py37hc64f4ea_0    anaconda
  mkl_random                1.1.1            py37h959d312_0    anaconda
  ncurses                   6.2                  h0a44026_1  
  numpy                     1.18.5           py37h1da2735_0    anaconda
  numpy-base                1.18.5           py37h3304bdc_0    anaconda
  openssl                   1.1.1g               h1de35cc_0    anaconda
  pandas                    1.0.5            py37h959d312_0    anaconda
  pip                       20.1.1                   py37_1  
  psycopg2                  2.8.4            py37ha12b0ac_0    anaconda
  pycodestyle               2.6.0              pyh9f0ad1d_0    conda-forge
  pycparser                 2.20                       py_0    conda-forge
  pycryptodome              3.9.7            py37h51495b9_1    conda-forge
  pyflakes                  2.2.0                      py_0    anaconda
  pylint                    2.5.3                    py37_0    anaconda
  pyparsing                 2.4.7              pyh9f0ad1d_0    conda-forge
  python                    3.7.6                h359304d_2  
  python-dateutil           2.8.1                      py_0    anaconda
  python_abi                3.7                     1_cp37m    conda-forge
  pytz                      2020.1                     py_0    anaconda
  readline                  7.0                  h1de35cc_5  
  setuptools                47.3.1                   py37_0  
  six                       1.15.0             pyh9f0ad1d_0    conda-forge
  sqlalchemy                1.3.17                   pypi_0    pypi
  sqlite                    3.32.3               hffcf06c_0  
  tk                        8.6.10               hb0a8c7a_0  
  toml                      0.10.1                     py_0    anaconda
  tornado                   6.0.4            py37h9bfed18_1    conda-forge
  typed-ast                 1.4.1            py37h1de35cc_0    anaconda
  werkzeug                  1.0.1                      py_0    anaconda
  wheel                     0.34.2                   py37_0  
  wrapt                     1.11.2           py37h1de35cc_0    anaconda
  wtforms                   2.3.1                      py_0    anaconda
  xlrd                      1.2.0                    py37_0    anaconda
  xz                        5.2.5                h1de35cc_0  
  zipp                      3.1.0                      py_0    anaconda
  zlib                      1.2.11               h1de35cc_3  
  </code></pre>
  <p>Finally I plan to deploy app in Heroku, so I thought that requirements.txt may be required.</p>