Python | sympy.subs() method

Last Updated : 18 Jun, 2019

With the help of sympy.subs() method, we can substitute all instances of a variable or expression in a mathematical expression with some other variable or expression or value.

Syntax: math_expression.subs(variable, substitute)
Parameters:
variable – It is the variable or expression which will be substituted.
substitute – It is the variable or expression or value which comes as substitute.
Returns: Returns the expression after the substitution.

Example #1:

In this example we can see that by using sympy.subs() method, we can find the resulting expression after substituting a variable or expression with some other variable or expression or value. Here we use symbols() method also to declare a variable as symbol.

# import sympy
from sympy import *
  
x, y = symbols('x y')
exp = x**2 + 1
print("Before Substitution : {}".format(exp)) 
    
# Use sympy.subs() method
res_exp = exp.subs(x, y) 
    
print("After Substitution : {}".format(res_exp)) 

Output:

 
Before Substitution : x**2 + 1
After Substitution : y**2 + 1

Example #2:

In this example we see that if the substituted value is numerical then sympy.subs() returns the solution of the resulting expression.

# import sympy
from sympy import * 
  
x = symbols('x')
exp = cos(x) + 7
print("Before Substitution : {}".format(exp)) 
    
# Use sympy.subs() method
res_exp = exp.subs(x, 0) 
    
print("After Substitution : {}".format(res_exp)) 

Output:

 
Before Substitution : cos(x) + 7
After Substitution : 8

Example #3:

In this example we see that we can do multiple substitutions if we pass a list of (old, new) pairs to subs.

# import sympy
from sympy import * 
  
x, y, z = symbols('x y z')
exp = x**2 + 7 * y + z
print("Before Substitution : {}".format(exp)) 
    
# Use sympy.subs() method
res_exp = exp.subs([(x, 2), (y, 4), (z, 1)]) 
    
print("After Substitution : {}".format(res_exp)) 

Output:

 
Before Substitution : x**2 + 7*y + z
After Substitution : 33

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