## 16.1.3 The Interactive Startup File 

When you use Python interactively, it is frequently handy to have some standard commands executed every time the interpreter is started. You can do this by setting an environment variable named$_{PYTHONSTARTUP}$ to the name of a file containing your start-up commands. This is similar to the .profile feature of the Unix shells. 

This file is only read in interactive sessions, not when Python reads commands from a script, and not when$_{/dev/}$ tty is given as the explicit source of commands (which otherwise behaves like an interactive session). It is executed in the same namespace where interactive commands are executed, so that objects that it defines or imports can be used without qualification in the interactive session. You can also change the prompts sys.ps1 and sys.ps2 in this file. 

If you want to read an additional start-up file from the current directory, you can program this in the global start- read()) . If you want to use the startup file in a script, you must do this explicitly in the script: 

import os filename=os.environ.get('PYTHONSTARTUP ') if filename and os.path.isfile(filename): with open(filename) as fobj: startup_file=fobj.read() exec(startup_file) 

## 16.1.4 The Customization Modules 

Python provides two hooks to let you customize it:$_{sitecustomize}$ and$_{usercustomize}$. To see how it works, you need first to find the location of your user site-packages directory. Start Python and run this code: 

>>> import site >>> site.getusersitepackages() '/home/user/.local/lib/python3.5/site-packages' 

Now you can create a file named$_{usercustomize.py}$in that directory and put anything you want in it. It will affect every invocation of Python, unless it is started with the$_{-s}$option to disable the automatic import. 

sitecustomize works in the same way, but is typically created by an administrator of the computer in the global site-packages directory, and is imported before$_{usercustomize}$. See the documentation of the$_{site}$ module for more details. 

