code works on desktop python v3.7.0 but not on micropython v1.9.4

[Xpl0itR]

I wrote some code to automate Euler's Method for Differential Equations. It works on desktop python v3.7.0 but not on micropython v1.9.4

Code: Select all

def function(x, y, h):
    derivalue = float(eval(dydx))
    print("|| "+str(round(x ,2))+" || "+str(round(y ,2))+" || "+str(round(derivalue ,2))+" || "+str(round(x+h ,2))+" || "+str(round(y+(derivalue*h) ,2))+" ||")
    global xCoord
    global yCoord
    xCoord = x + h
    yCoord = y+(derivalue*h)

iterations = int(input("Iterations: "))
xCoord = float(input("Initial x coordinate: "))
yCoord = float(input("Initial y coordinate: "))
hSteps = float(input("Step size: "))
dydx=input("dy/dx=")

print("==============================================")
print("||  X  ||  Y  || dy/dx || x+h || y+(dy/dx)h ||")
print("==============================================")

for i in range(iterations):
    function(xCoord, yCoord, hSteps)

if i enter:

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Iterations: 4
Initial x coordinate: 1
Initial y coordinate: 1
Step size: 0.5
dy/dx=x**2 + y**2

on Desktop python i get:

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===========================
||  X  ||  Y  || dy/dx || x+h || y+(dy/dx)h ||
===========================
|| 1.0 || 1.0 || 2.0 || 1.5 || 2.0 ||
|| 1.5 || 2.0 || 6.25 || 2.0 || 5.12 ||
|| 2.0 || 5.12 || 30.27 || 2.5 || 20.26 ||
|| 2.5 || 20.26 || 416.63 || 3.0 || 228.57 ||

but on Micropython i get:

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===========================
||  X  ||  Y  || dy/dx || x+h || y+(dy/dx)h ||
===========================
Traceback (most recent call last):
  File "<stdin>", line 20, in <module>
  File "<stdin>", line 2, in function
  File "<string>", line 1, in <module>
NameError: name 'x' is not defined

What am i doing wrong?

[fstengel]

The problem lies with eval: it only works in the global scope. Due to micropython's implementation, the only names it can see are the global ones. See post 4 in this thread

[Roberthh]

The locals() dictionary is identical to globals(). Even if that is documented, I would consider that as a surprising omission. As a workaround you can supply a specific dictionary to your eval() call, like

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    derivalue = float(eval(dydx, globals(), {"x":x, "y":y}))

[fstengel]

It is documented there. I gather it is one of the compromises one has to live with when using micropython.

[Xpl0itR]

I got it working now, Thanks guys.