import pymysql.cursors

username = 'Bob'

# Fill in the details with your own connection information, obviously
connection = pymysql.connect(host='localhost',
                             user='user',
                             password='password',
                             db='db',
                             charset='utf8mb4',
                             cursorclass=pymysql.cursors.DictCursor)

# Grab a cursor that uses connection so the database can be interacted with
cursor = connection.cursor()

# Set up the query, indicating places for wildcards
query = 'SELECT * FROM Users WHERE name = %s'

# Run the query. The second argument is a tuple of arguments that will replace the wildcard(s).
# (Single arguments don't need the extra parentheses to define them as a tuple - pymysql can figure that out.)
num_results = cursor.execute(query, username)

# execute() returns the number of rows found, so you can just use its output value instead of fetchone() or fetchall()
if num_results > 0:
  print('Username is taken.')
else:
  print('Username is free.')