Eval vs. Exec vs. Compile in Python: What's the Difference?

Eval, Exec, and Compile: A Comparative Analysis

Differences in Functionality

Eval evaluates a single expression dynamically and returns the result.
Exec executes a given block of code and discards its return value, primarily used for its side effects.

Compile and Modes

Compile plays a crucial role in both eval and exec:

• 'eval' mode: Compiles a single expression into bytecode that returns the evaluated value.
• 'exec' mode: Compiles code fragments of any type into bytecode that returns None (in Python 2, it was a statement and did not return anything).

Syntax and Differences

Python 2

• Exec is a statement.
• Eval is a function.
• Exec accepts a tuple of length 2 or 3 to define globals and locals (a hack for backward compatibility).

Python 3

• Exec is a function.
• Exec and eval behave identically when passed a code object.

Detailed Comparison

Evaluating Expressions:

• Exec does not accept expressions directly.
• Eval accepts only single expressions as strings or code objects.

Side Effects:

• Exec is used solely for side effects like variable modifications or function calls.
• Eval has no side effects, returning the value of the evaluated expression.

Statements and Code Blocks:

• Exec accepts code blocks containing statements, loops, and imports.
• Eval cannot handle statements directly; it requires compiled code objects for statements.

Example Usage

Calculating and Printing:

a = 5
result = eval('37 + a')   # Eval calculates the expression and returns the result (42)
exec('print(37 + a)')     # Exec executes the code (prints 42)

Modifying Variables:

a = 2
exec('a = 47')            # Exec modifies the global variable `a`
result = eval('a = 47')     # Eval throws an error because it cannot handle statements

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