Editing a Python script from PHP - php

Question

I want to edit the code of a Python script from PHP, to change a formula and initial value for one of my projects.
The Python script:
import numpy as np
import scipy as sp
import math
from scipy.integrate import odeint
import matplotlib.pyplot as plt
def g(y, x):
y0 = y[0]
y1 = x #formula
return y1
init = 90#formula2
x= np.linspace(0,1,100)
sol=odeint(g, init, x)
plt.legend()
plt.plot(x, sol[:,0], color='b')
plt.show()
PHP:
<?php
if (isset($_POST['equation1'])&&isset($_POST['constant'])) {
$eqn1 = $_POST['equation1'];
//$equation = $_POST['equation'];
$eqn2 = $_POST['constant'];
//str_replace("^", "**", $equation);
$filename ='ode3.py';
$pgm = "C:\\wamp\\www\\working\\".$filename;
function get_string_between($string, $start, $end) {
$string = " ".$string;
$ini = strpos($string,$start);
if ($ini == 0) return "";
$ini += strlen($start);
$len = strpos($string,$end,$ini) - $ini;
return substr($string,$ini,$len);
}
//write the equation in python code
$pgm_file = $pgm;
$myfile = fopen($pgm_file, "r") or die("Unable to open file!");
$data = fread($myfile,filesize($pgm_file));
$parsed = get_string_between($data, "y1 = ", " #formula");
$parsed2 = get_string_between($data, "init = ", "#formula2");
$datanew1 = str_replace($parsed, $eqn1, $data);
$datanew2 = str_replace($parsed2,$eqn2, $datanew1);
fclose($myfile);
$myfile = fopen($pgm_file, "w") or die("Unable to open file!");
$datacode = fwrite($myfile,$datanew2);
$pgmfile = fopen($pgm_file, "r") or die("Unable to open file!");
$pgmdata = fread($pgmfile,filesize($pgm_file));
fclose($pgmfile);
/*$demo = fopen($pgm, "r") or die("Unable to open file!");
$xxx = fread($demo,filesize($pgm));
echo $xxx;*/
$pyscript = "C:\\wamp\\www\\working\\".$filename;
$python = 'C:\\Python34\\python.exe';
$cmd = "$python $pyscript";
exec("$cmd", $output);
/* if (count($output) == 0) {
echo "error%";
} else {
echo $output[0]."%";*/
}
var_dump($output);
}
?>
I want it to only replace the part of the Python script marked by #formula and #formula2, where x should be replaced by x+45 and the init value should be set to 85.
However, when I run the PHP script, every instance of x in the Python script is replaced with x+45. How can I limit it to only the part marked by #formula inside the g() function?

Answer 1

I think this is a poor way to approach this problem. Rather than editing the source of your Python script, you should re-engineer it so that the variable parts can be passed in as arguments when the script is called. You can then access these execution arguments using sys.argv.
#!/usr/bin/env python
import sys
# ... import numpy etc.
def g(y,x, expr):
y0 = y[0]
# use the Python interpreter to evaluate the expression.
# It should be of the form 'x**2 + 3' or something - a valid Python/math
# expression otherwise it will crash with a syntax error.
y1 = eval(expr)
return y1
if __name__=='__main__':
init = int(sys.argv[1]) # the first argument passed to the script
expr = sys.argv[2].strip('"') # the second argument - if it includes
# whitespace, wrap it in "" so it works.
# ... do stuff
You can then call this as a standard script using PHP's exec or shell_exec function. Pass the function expression in as arguments like so:
python /path/to/ode3.py 85 "x+45".
Please be very careful with Python's eval - it will evaluate anything you put in there. Since you are calling this from a web-facing PHP script that accepts POST arguments, you must be careful to sanitize the input. From the script you've posted, it doesn't look like you validate it, so if it is publicly accessible, it will allow arbitrary code execution with the same permissions as the PHP script. This is true whether you use eval or edit the Python file as you have been attempting.
Update: Also, the operators have different meanings in different languages. For example, in Python, the ^ operator does not mean exponent, it actually means bitwise XOR. If you want to get x^2 (x-squared), the Python syntax is x**2. If you want to be able to write x^2, you would need to write a parser. See here for more information.