Python compile() Method

The Python compile() function is a built-in function that compiles source code into a code object that can be executed by exec() or evaluated by eval(). It accepts six parameters: source (a string, byte string, or AST object containing the code), filename (a string identifying the source file for error messages), mode (one of 'exec', 'eval', or 'single' depending on the type of code), and optional flags, dont_inherit, and optimize parameters for controlling compilation behavior. The function returns a code object that Python’s virtual machine can execute. The compile() function is useful in scenarios where code needs to be parsed and validated before execution, such as building template engines, interactive interpreters, or educational tools that evaluate user-submitted Python code. It allows you to check for syntax errors at compile time rather than runtime. It works closely with exec() for executing statements and eval() for evaluating expressions.

What does compile() return?

The compile() function returns a Python code object that represents the compiled source code and can be passed to exec() or eval() for execution.

When should you use compile()?

Use compile() when you need to compile Python source code from a string or file for repeated execution, or when you want to validate code syntax before running it.

The compile() method in python code objects from the source can be a normal string, a byte string, or an AST object.

The syntax of compile() is:

compile(source, filename, mode, flags=0, dont_inherit=False, optimize=-1)

Python compile() Method Parameters

• source - A simple string, a byte string, or an AST object.
• filename - The name of the file from the code comes from. If the code does not come from a file you can write anything you like.

• mode - Legal values can be eval, or exec, or single.

  • eval - Accepts only a single expression.
  • exec - It can take a code block with a python statement, class, and methods, etc.
  • single - if it consists of a single interactive statement.
• flags - How to compile the source. The default will be 0.

• don’t-inherit - How to compile the source. Default False

• optimize - Defines the optimization level of the compiler. Default -1

The compile() method is used if the python code is in simple string form or is an AST object, and if you want to change it to a code object.

Let us see an example of the usage of compile() method.

Example 1: How to use compile() method?

codeWithString = 'x = 7\ny=12\nsum=x+y\nprint("sum =",sum)'
CodeObject = compile(codeWithString,'substring','exec')
exec(CodeObject)
print(type(CodeObject))

Output will be as follow:

sum = 19
<class 'code'>

Here we are taking a simple string as a source. And the filename is sumstring. After that, we are taking exec mode that allows us the usage of exec() method.

As we can see, it showing class code when we are checking its type.

In simple words compile() method can convert the simple string to a python code object. The code object can be executed using the exec() method.

Example 2: How to use compile() method with eval()?

Let see an example to use the compile() method to execute the code using the eval() method.

X = 10

CodeEx = compile('X  == 10','','eval')
CodeObject = eval(CodeEx)
print(CodeObject)

CodeEx = compile('X  + 10','','eval')
CodeObject = eval(CodeEx)
print(CodeObject)

The output will be as follow:

True
20

Example 3: How to use compile() method byte string source?

Let take an example of using byte string as source in compile() method.

x = 10
byte_string = bytes('x == 10','utf-8')
CodeObject = compile(byte_string,'','eval')
result = eval(CodeObject)
print(result)
print(type(byte_string))

Output:

True
<class 'bytes'>

Common Use Cases

A common use case for compile() is building template engines that compile template expressions once and execute them repeatedly against different data contexts, improving performance over re-parsing each time. Another practical scenario is creating educational tools or online coding platforms that compile user-submitted code to check for syntax errors before executing it in a sandboxed environment. It is also used in dynamic configuration systems where business rules are stored as Python expressions in a database and compiled at startup for efficient repeated evaluation.

Rules of compile() method

compile() method returns objects as python code object.