What are the Differences Between Python's `eval`, `exec`, and `compile` Functions?

The Difference Between eval, exec, and compile

Introduction

Python provides three powerful tools for executing code dynamically: eval, exec, and compile. Each serves a specific purpose, and it's important to understand their differences.

eval

Eval is used to evaluate a single expression. It takes a string representing the expression and returns its value. For example:

a = 5
result = eval('a + 10')
print(result)  # Output: 15

exec

Exec is used to execute a block of code. It takes a string representing the code and returns None. For example:

my_code = """
for i in range(5):
    print(i)
"""

exec(my_code)
# Output:
# 0
# 1
# 2
# 3
# 4

compile

Compile is used to compile code into bytecode, which can then be executed by the interpreter. It takes a string representing the code and a mode ('eval', 'exec', or 'single').

In 'eval' mode, compile compiles a single expression into bytecode that returns the expression's value. In 'exec' mode, it compiles a block of code into bytecode that returns None. In 'single' mode, it compiles a single statement or expression into bytecode that prints the repr of the expression's value.

For example:

bytecode = compile('a + 10', 'my_code', 'eval')
result = eval(bytecode)
print(result)  # Output: 15

bytecode = compile("""
for i in range(5):
    print(i)
""", 'my_code', 'exec')
exec(bytecode)
# Output:
# 0
# 1
# 2
# 3
# 4

Summary

Function	What it does
eval	Evaluates a single expression
exec	Executes a block of code
compile	Compiles code into bytecode

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