What does the second argument of the Session.pop method do in Python Flask?

Question

I'm working through the Flask tutorial and would just like to clarify exactly what the .pop attr of the session object does and why it would take a 'None' parameter.

@app.route('/logout')
def logout():
    session.pop('logged_in', None)
    flash('You were logged out')
    return redirect(url_for('show_entries'))

Answer 1

According to Flask's API their Session class is a wrapper around a python Dict. According to the python documentation for dict.pop():

pop(key[, default])

If key is in the dictionary, remove it and return its value, else return default. If default is not given and key is not in the dictionary, a KeyError is raised.

In this case the tutorial asks you to pass in None as the default value.