How can I make a python script change itself?

Question

To boil it down, I would like to have a python script (run.py)like this

a = 0
b = 1
print a + b
# do something here such that the first line of this script reads a = 1

Such that the next time the script is run it would look like

a = 1
b = 1
print a + b
# do something here such that the first line of this script reads a = 2

Is this in any way possible? The script might use external resources; however, everything should work by just running the one run.py-file.

EDIT: It may not have been clear enough, but the script should update itself, not any other file. Sure, once you allow for a simple configuration file next to the script, this task is trivial.

Yes it is possible. In fact there are several ways of achieving that goal. — Tomasz Plaskota
– Tomasz Plaskota, Commented Sep 22, 2016 at 15:47
You can open up your script with open('run.py','w')... and make alterations to it, if that's what you want. Is that all you wanted to know? — khelwood
– khelwood, Commented Sep 22, 2016 at 15:50
The script name is in __file__ (see stackoverflow.com/questions/4152963/…) and you can open it like any other file. However, if all you want to do is to save data between runs there are much better ways to do it. — cdarke
– cdarke, Commented Sep 22, 2016 at 15:51
@khelwood: If you open the original as write then how will the OP get the lines to write? The script need to be opened as read, then copied line-by-line to a new file (opened as write) altering those lines required. Then rename the copy. — cdarke
– cdarke, Commented Sep 22, 2016 at 16:00

Gerrat · Accepted Answer · 2023-08-15 13:15:56Z

14

For an example (changing the value of a each time its run):

a = 0
b = 1
print(a + b)

with open(__file__, 'r') as f:
    lines = f.read().split('\n')
    val = int(lines[0].split('=')[-1])
    new_line = 'a = {}'.format(val+1)
    new_file = '\n'.join([new_line] + lines[1:])

with open(__file__, 'w') as f:
    f.write(new_file)

edited Aug 15, 2023 at 13:15

answered Sep 22, 2016 at 16:02

Gerrat

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Comments

Joe · Accepted Answer · 2016-09-22 18:30:37Z

8

What you're asking for would require you to manipulate files at the {sys} level; basically, you'd read the current file in, modify it, over-write it, and reload the current module. I played with this briefly because I was curious, but I ran into file locking and file permission issues. Those are probably solvable, but I suspect that this isn't really what you want here.

First: realize that it's generally a good idea to maintain a separation between code and data. There are exceptions to this, but for most purposes, you'll want to make the parts of your program that can change at runtime read their configuration from a file, and write changes to that same file.

Second: idomatically, most python projects use YAML for configuration

Here's a simple script that uses the yaml library to read from a file called 'config.yaml', and increments the value of 'a' each time the program runs:

#!/usr/bin/python
import yaml

config_vals = ""
with open("config.yaml", "r") as cr:
   config_vals = yaml.load(cr)

a = config_vals['a']
b = config_vals['b']
print a + b

config_vals['a'] = a + 1
with open("config.yaml", "w") as cw:
   yaml.dump(config_vals, cw, default_flow_style=True)

The runtime output looks like this:

$ ./run.py
3 
$ ./run.py
4
$ ./run.py
5

The initial YAML configuration file looks like this:

a: 1
b: 2

answered Sep 22, 2016 at 18:30

Joe

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1 Comment

ImportanceOfBeingErnest Over a year ago

Actually, the thing you're describing in the first paragraph is exactly what I want.

TigerhawkT3 · Accepted Answer · 2016-09-22 16:02:44Z

2

Make a file a.txt that contains one character on one line:

Then in your script, open that file and retrieve the value, then immediately change it:

with open('a.txt') as f:
    a = int(f.read())
with open('a.txt', 'w') as output:
    output.write(str(a+1))
b = 1
print a+b

On the first run of the program, a will be 0, and it will change the file to contain a 1. On subsequent runs, a will continue to be incremented by 1 each time.

answered Sep 22, 2016 at 16:02

TigerhawkT3

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Comments

gz200 · Accepted Answer · 2023-01-19 11:51:37Z

0

Gerrat's code but modified.

#some code here
a = 0
b = 1

print(a + b)

applyLine = 1#apply to wich line(line 1 = 0, line 2 = 1)
with open(__file__, 'r') as f:
  lines = f.read().split('\n')#make each line a str in a list called 'lines'
  val = int(lines[applyLine].split(' = ')[-1])#make an int to get whatever is after ' = ' to applyed line 
  new_line = 'a = {}'.format(val+1)#generate the new line
lines[applyLine] = new_line#update 'lines' to add the new line
write = "\n".join(lines)#create what to rewrite and store it in 'write' as str
with open(__file__, 'w') as f:
  f.write(write)#update the code

answered Jan 19, 2023 at 11:51

gz200

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2 Comments

Community Over a year ago

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Michael Katt Over a year ago

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