show "(env)" at start of Terminal prompt when Python virtual environment env is activated

[dinhhnguyen]

with Installation Instructions 4. in your homepage

# in .zshrc

precmd() {
  __posh_git_ps1 '%n@%m:%~ ' '$ '
}

my Terminal prompt shows

dinh@serial_number:~/git_repository [git status]$

But when I activate the repository's Python virtual environment env, it does not show "(env)" at the start of prompt as below

(env) dinh@serial_number:~/git_repository [git status]$

How can I change the above precmd(), or what can I do, to get that "(env)" when activated?
Thanks.