Python eval() Function
Python3 Built-in Functionseval()Is a built-in function in Python used to execute Python expressions in strings.
eval()Accepts a string, executes it as Python code, and returns the execution result. It is a powerful feature, but improper use can bring security risks.
Word Definition： evalYesevaluateAbbreviation of (evaluation).
Basic Syntax and Parameters
Syntax Format
eval(expression, globals, locals)
Parameter Description
- Parameter expression：
- Type: String
- Description: The Python expression to be executed.
- Parameter globals(Optional):
- Type: Dictionary
- Description: Global namespace.
- Parameter locals(Optional):
- Type: Dictionary
- Description: Local namespace.
Function Description
- Return Value: Returns the execution result of the expression.
- Security Warning: Do not use on untrusted input
eval(), as it may lead to code injection attacks.
Examples
Example 1: Basic Usage
Example
# Calculate a mathematical expression
result = eval("1 + 2 + 3")
print(result) # Output: 6
# Using variables
x = 10
result = eval("x * 2")
print(result) # Output: 20
# Complex expression
result = eval("2 ** 3 + 4 * 5")
print(result) # Output: 28
# Function call
result = eval("len('hello')")
print(result) # Output: 5
result = eval("1 + 2 + 3")
print(result) # Output: 6
# Using variables
x = 10
result = eval("x * 2")
print(result) # Output: 20
# Complex expression
result = eval("2 ** 3 + 4 * 5")
print(result) # Output: 28
# Function call
result = eval("len('hello')")
print(result) # Output: 5
Expected output:
6 20 28 5
Code explanation:
eval()Can execute arithmetic expressions.- Can reference defined variables.
- Can call built-in functions.
Example 2: Restricted Environment
Example
# Restrict available functions and variables
x = 10
y = 20
# Create a restricted global dictionary
safe_dict = {"x": x, "y": y, "abs": abs}
result = eval("x + y", safe_dict)
print(result) # Output: 30
# Disallow access to dangerous functions
try:
eval("__import__('os').system('ls')", {})
except NameError as e:
print(f"Security restriction: {e}")
x = 10
y = 20
# Create a restricted global dictionary
safe_dict = {"x": x, "y": y, "abs": abs}
result = eval("x + y", safe_dict)
print(result) # Output: 30
# Disallow access to dangerous functions
try:
eval("__import__('os').system('ls')", {})
except NameError as e:
print(f"Security restriction: {e}")
Expected output:
30 安全限制: name '__import__' is not defined
This example demonstrates how to improveeval()security by restricting the globals dictionary.
Other Extensions