input: Write a function to modify the provided Python code so that it prompts the user to enter the value of the radius to calculate the area of a circle. The function should return the modified code and pass the following assertions.
  assert modify_code("")=='\n# Calculate area of a circle\n\nPi = 3.14\n\n# Get radius from user\nradius = float(input("Please enter the radius of the circle: "))\n\n# Calculate the circle area\narea = Pi*radius**2\n\n# Print the area\nprint("The area of the circle is:", area)'
  assert modify_code("# Placeholder code")=='\n# Calculate area of a circle\n\nPi = 3.14\n\n# Get radius from user\nradius = float(input("Please enter the radius of the circle: "))\n\n# Calculate the circle area\narea = Pi*radius**2\n\n# Print the area\nprint("The area of the circle is:", area)'
output: def modify_code(code):
      return '\n# Calculate area of a circle\n\nPi = 3.14\n\n# Get radius from user\nradius = float(input("Please enter the radius of the circle: "))\n\n# Calculate the circle area\narea = Pi*radius**2\n\n# Print the area\nprint("The area of the circle is:", area)'