To download source code of a package, first add or uncomment the deb-src lines in /etc/apt/sources.list. The run sudo apt-get update to make the package manager obtain available package sources. Use apt-get source <package_name> to download the source, bu be noticed this will extract the code for tarballs.

vi /etc/apt/sources/lis # add what need
sudo apt-get update
apt-get source python #example

Use = to specify a version: apt-get source cmake=3.5, or use --comiple to compile the package to a binary .deb: apt-get source python --compile.

For apt all work the same way:

apt source python

But some packages source could not be found with apt-get e.g. bison. Try to use apt source bison to ge the source code works in my case.(why?)