how to translate the following Applescript to appscript:
tell application "System Events"
key code 0 using command down
end tell
I want to perform "Command + A"-like short cut, i.e., select all texts.
Look at the application ASTranslate which was installed as part of Appscript. It translates Applescript to Appscript for Python or Ruby. Be aware it just traps Apple Events and thus won't translate Applescript structures like loops or the like. It's very easy to use. Just past your Applescript in one pane, hit cmd-R, and it'll generate the translated Appscript + Python code. For your example it is
app(u'System Events').key_code(0, using=k.command_down)
While occasionally you'll find something that won't work quite right in general ASTranslate is an essential tool for using Appscript.