Evaluate boolean expression

# Table Variables:
# 
# +---------------+---------+
# | Column Name   | Type    |
# +---------------+---------+
# | name          | varchar |
# | value         | int     |
# +---------------+---------+
# In SQL, name is the primary key for this table.
# This table contains the stored variables and their values.
# 
#  
# 
# Table Expressions:
# 
# +---------------+---------+
# | Column Name   | Type    |
# +---------------+---------+
# | left_operand  | varchar |
# | operator      | enum    |
# | right_operand | varchar |
# +---------------+---------+
# In SQL, (left_operand, operator, right_operand) is the primary key for this table.
# This table contains a boolean expression that should be evaluated.
# operator is an enum that takes one of the values ('<', '>', '=')
# The values of left_operand and right_operand are guaranteed to be in the Variables table.
# 
#  
# 
# Evaluate the boolean expressions in Expressions table.
# 
# Return the result table in any order.
# 
# The result format is in the following example.
#
# Example 1:
# Input:
# Variables table:
# +------+-------+
# | name | value |
# +------+-------+
# | x    | 66    |
# | y    | 77    |
# +------+-------+
# Expressions table:
# +--------------+----------+---------------+
# | left_operand | operator | right_operand |
# +--------------+----------+---------------+
# | x            | >        | y             |
# | x            | <        | y             |
# | x            | =        | y             |
# | y            | >        | x             |
# | y            | <        | x             |
# | x            | =        | x             |
# +--------------+----------+---------------+
# Output:
# +--------------+----------+---------------+-------+
# | left_operand | operator | right_operand | value |
# +--------------+----------+---------------+-------+
# | x            | >        | y             | false |
# | x            | <        | y             | true  |
# | x            | =        | y             | false |
# | y            | >        | x             | true  |
# | y            | <        | x             | false |
# | x            | =        | x             | true  |
# +--------------+----------+---------------+-------+
# Explanation:
# As shown, you need to find the value of each boolean expression in the table using the variables table.

import pandas as pd

def eval_expression(variables: pd.DataFrame, expressions: pd.DataFrame) -> pd.DataFrame:
  var_map = dict(zip(variables['name'], variables['value']))
  expressions['left_val'] = expressions['left_operand'].map(var_map)
  expressions['right_val'] = expressions['right_operand'].map(var_map)
  
  def evaluate(row):
    if row['operator'] == '>':
      return 'true' if row['left_val'] > row['right_val'] else 'false'
    elif row['operator'] == '<':
      return 'true' if row['left_val'] < row['right_val'] else 'false'
    else:
      return 'true' if row['left_val'] == row['right_val'] else 'false'
  
  expressions['value'] = expressions.apply(evaluate, axis=1)
  return expressions[['left_operand', 'operator', 'right_operand', 'value']]