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PODefining a global function in a Python script
 

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  1. PODefining a global function in a Python script
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    <p>I'm new to Python. I am writing a script that will numerically integrate a set of ordinary differential equations using a Runge-Kutta method. Since the Runge-Kutta method is a useful mathematical algorithm, I've put it in its own .py file, rk4.py.</p> <pre><code>def rk4(x,dt): k1=diff(x)*dt k2=diff(x+k1/2)*dt k3=diff(x+k2/2)*dt k4=diff(x+k3)*dt return x+(k1+2*k2+2*k3+k4)/6 </code></pre> <p>The method needs to know the set of equations that the user is working with in order to perform the algorithm, so it calls a function <code>diff(x)</code> that will find give rk4 the derivatives it needs to work. Since the equations will change by use, I want diff() to be defined in the script that will run the particular problem. In this case the problem is the orbit of mercury, so I wrote <code>mercury.py</code>. (This isn't how it will look in the end, but I've simplified it for the sake of figuring out what I'm doing.)</p> <pre><code>from rk4 import rk4 import numpy as np def diff(x): return x def mercury(u0,phi0,dphi): x=np.array([u0,phi0]) dt=2 x=rk4(x,dt) return x mercury(1,1,2) </code></pre> <p>When I run mercury.py, I get an error:</p> <pre><code> File "PATH/mercury.py", line 10, in mercury x=rk4(x,dt) File "PATH/rk4.py", line 2, in rk4 k1=diff(x)*dt NameError: global name 'diff' is not defined </code></pre> <p>I take it since <code>diff()</code> is not a global function, when rk4 runs it knows nothing about diff. Obviously rk4 is a small piece of code and I could just shove it into whatever script I'm using at the time, but I think a Runge-Kutta integrator is a fundamental mathematical tool, just like the array defined in NumPy, and so it makes sense to make it a function that is called rather one that is defined in every script that uses it (which may be many). But I also can't go telling rk4.py to import a particular diff from a particular .py file, because that ruins the generality of rk4 that I want in the first place.</p> <p>Is there a way to define diff globally within a script like mercury.py so that when rk4 is called, it will know about diff?</p>
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    1. USDylan B
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    1. COFix your indentation please.
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    2. COBy the way, the whitespace in the block quotes is not accurate to my code, but I'm not sure how to get the block quotes to generate a new line without the extra whitespace.
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    3. COUse the `{ }` icon in the question editor to mark large blocks of source code.
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