I'm having trouble using the subs command in Python using the Sympy module. It's like the command doesn't exist. The question in the code is about the chain rule when x and y depend on a common variable.

Here is my code:

import sympy as sp
from sympy import E
import math

x1, y1, t1 = sp.symbols('x y t')

# Question 2

# Method 1

pi = math.pi

f2 = E**y1 * sp.cos(x1)

dfdx = sp.diff(f2, x1)

dfdy = sp.diff(f2, y1)

xt = (2*pi / 3)*t1 + pi/4
dxdt = sp.diff(xt, t1)

yt = -t1
dydt = sp.diff(yt, t1)

dfdt = dfdx *  dxdt + dfdy * dydt
print(dfdt.subs({x1 : xt}, {y1 : yt}))

My output is just the original dfdt in terms of x and y and not in terms of t:

PS C:\Users\maxko\OneDrive\Pulpit\Python> & C:/Users/maxko/AppData/Local/Programs/Python/Python311/python.exe "c:/Users/maxko/OneDrive/Pulpit/Python/Lab 2.py"
-2.0943951023932*exp(y)*sin(x) - exp(y)*cos(x)

Follow up question, is there any way to prettify that long float? Maybe keep it in terms of pi or just 3 sig figs?

I was expecting for the subs command to substitute the 'x' and 'y' in the 'dfdt' for the 'xt' and 'yt' that were defined above.

I'm having trouble using the subs command in Python using the Sympy module. It's like the command doesn't exist. The question in the code is about the chain rule when x and y depend on a common variable.

Here is my code:

import sympy as sp
from sympy import E
import math

x1, y1, t1 = sp.symbols('x y t')

# Question 2

# Method 1

pi = math.pi

f2 = E**y1 * sp.cos(x1)

dfdx = sp.diff(f2, x1)

dfdy = sp.diff(f2, y1)

xt = (2*pi / 3)*t1 + pi/4
dxdt = sp.diff(xt, t1)

yt = -t1
dydt = sp.diff(yt, t1)

dfdt = dfdx *  dxdt + dfdy * dydt
print(dfdt.subs({x1 : xt}, {y1 : yt}))

My output is just the original dfdt in terms of x and y and not in terms of t:

PS C:\Users\maxko\OneDrive\Pulpit\Python> & C:/Users/maxko/AppData/Local/Programs/Python/Python311/python.exe "c:/Users/maxko/OneDrive/Pulpit/Python/Lab 2.py"
-2.0943951023932*exp(y)*sin(x) - exp(y)*cos(x)

Follow up question, is there any way to prettify that long float? Maybe keep it in terms of pi or just 3 sig figs?

I was expecting for the subs command to substitute the 'x' and 'y' in the 'dfdt' for the 'xt' and 'yt' that were defined above.

• python
• math
• sympy

1 Answer 1

You arguments for subs are incorrect. You should pass a single dictionary with the key-value pairs.

print(dfdt.subs({x1: xt, y1: yt}))

That will give you your desired substitution.

Regarding the pi part of your question, try not to mix numerical and symbolic libraries. You use math.pi when you should probably use sympy's pi (sp.pi, in your case). Then you don't have to worry about the decimals of the printout.