# Python | sympy.subs() method

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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