<human>:  expand variable in string.
<bot>: Here is a Python function to expand variable in string

import string

def expand_variable(string, variable):
  """Expand variable in string."""
  return string.replace(variable, '${' + variable + '}')

# Enter the string
string = "Hello"

# Enter the variable
variable = "world"

# Calling the function
print((expand_variable(string, variable)))

#Example Output
# Hello

#Remember to inspect code for bugs.