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compile() Function Complexity

The compile() function turns Python source, or an ast tree, into a code object that eval() and exec() run. It tokenizes, parses, builds a symbol table and emits bytecode, and each stage is linear in the source. Nothing is cached: compiling the same source twice compiles it twice, and eval() or exec() given a string compiles it on every call.

n is the length of the source: characters in a str, bytes in a bytes. t is the nodes in an ast tree, counting each item of a tuple or frozenset held by an ast.Constant. Running the code object is not part of compile(); its cost is whatever the compiled code does.

Complexity Reference

compile()

Operation	Time	Space	Notes
compile(source, filename, mode) with a str or bytes source	O(n)	O(n)	No cache: the same source compiled again costs O(n) again. Constant folding has size limits, so a short expression such as 'a' * 10**8 is left for run time rather than built while compiling
mode='exec', mode='eval', mode='single'	O(n)	O(n)	'exec' takes statements, 'eval' one expression, 'single' one interactive statement whose expression values are printed
compile(source, filename, mode, flags=ast.PyCF_ONLY_AST)	O(n)	O(n)	Stops after parsing and returns the ast tree, as ast.parse() does
compile(tree, filename, mode) with an ast tree	O(t)	O(t)	The tree is validated first; nodes built by hand need line numbers, which ast.fix_missing_locations() fills in
optimize=-1, 0, 1, 2	O(n)	O(n)	-1 follows the interpreter's -O; 1 removes assert statements and makes __debug__ false; 2 also removes docstrings
flags, dont_inherit	O(n)	O(n)	Future features and ast.PyCF_* options. By default the caller's from __future__ imports apply as well; dont_inherit=True uses flags alone
Invalid source	O(n)	O(n)	Raises SyntaxError with filename and the line, scoping errors such as a nonlocal with no binding included; an undefined name is not an error until the code runs

Running the result

Operation	Time	Space	Notes
eval(code), exec(code)	The code's own cost	The code's own cost	No compilation; see eval() and exec()
eval(string), exec(string)	O(n) + the code's cost	O(n) + the code's cost	Compiles the string on every call

Compile Once, Run Many Times

eval() and exec() given a string compile it every time. Compiling it once and passing the code object leaves only the run. An audit hook shows each compilation as it happens.

import sys

compiled = []
sys.addaudithook(lambda event, args: compiled.append(args[1]) if event == "compile" else None)

x = 3
for _ in range(100):
    assert eval("x ** 2") == 9  # O(n) per call - compiles the string each time
assert compiled.count("<string>") == 100

code = compile("x ** 2", "<expr>", "eval")  # O(n), once
for _ in range(100):
    assert eval(code) == 9  # no compilation, only the expression runs
assert compiled.count("<expr>") == 1

Modes

The mode decides what the source may contain, not what compiling it costs. 'single' is the interactive prompt's mode: an expression statement prints its value unless it is None.

import contextlib
import io

code = compile("2 + 3", "<string>", "eval")  # O(n) - one expression
assert eval(code) == 5

code = compile("x = 10\ny = 20\nz = x + y\n", "<string>", "exec")  # O(n) - statements
namespace = {}
exec(code, namespace)
assert namespace["z"] == 30

code = compile("1 + 1", "<string>", "single")  # O(n) - one interactive statement
output = io.StringIO()
with contextlib.redirect_stdout(output):
    exec(code)
assert output.getvalue() == "2\n"

try:
    compile("x = 1\ny = 2", "<string>", "single")
except SyntaxError as error:
    assert "multiple statements" in str(error)
else:
    raise AssertionError("'single' accepted two statements")

Sources and Trees

A bytes source is decoded first, honouring a coding declaration, and costs O(n) in its bytes. ast.PyCF_ONLY_AST stops after parsing and returns the tree; a tree passed back in is compiled without parsing, so a program can parse once, transform the tree, and compile the result.

import ast

code = compile(b"# -*- coding: latin-1 -*-\nname = '\xe9'\n", "<bytes>", "exec")  # O(n)
namespace = {}
exec(code, namespace)
assert namespace["name"] == "é"

tree = compile("total = price * count", "<string>", "exec", flags=ast.PyCF_ONLY_AST)  # O(n)
assert ast.dump(tree) == ast.dump(ast.parse("total = price * count"))

tree.body[0].value.op = ast.Add()  # turn the product into a sum
code = compile(tree, "<ast>", "exec")  # O(t)
namespace = {"price": 3, "count": 4}
exec(code, namespace)
assert namespace["total"] == 7

Optimization Levels

optimize changes what is emitted, not the bound. Level 1 drops assert statements and compiles __debug__ as false; level 2 also drops docstrings.

source = '''
def check(value):
    "Return value, which must be positive."
    assert value > 0
    return value
'''

namespace = {}
exec(compile(source, "<string>", "exec", optimize=0), namespace)  # O(n)
assert namespace["check"].__doc__ == "Return value, which must be positive."
try:
    namespace["check"](-1)
except AssertionError:
    pass
else:
    raise AssertionError("optimize=0 removed the assert")

exec(compile(source, "<string>", "exec", optimize=1), namespace)  # O(n)
assert namespace["check"](-1) == -1  # the assert is gone
assert namespace["check"].__doc__ is not None

exec(compile(source, "<string>", "exec", optimize=2), namespace)  # O(n)
assert namespace["check"].__doc__ is None  # and so is the docstring

Future Features

flags takes a feature's compiler_flag from the __future__ module. Without dont_inherit=True, the features in force where compile() is called apply too.

import __future__

source = "def area(width: float, height: float) -> float: return width * height"

code = compile(source, "<string>", "exec", flags=__future__.annotations.compiler_flag)
namespace = {}
exec(code, namespace)
assert namespace["area"].__annotations__ == {
    "width": "float", "height": "float", "return": "float"
}

Limits on Nesting

Nesting depth has hard limits. More than 200 open brackets raises SyntaxError, and a block indented 100 levels deep raises IndentationError. A chain the grammar nests - a + b + c + ..., or an if with a long run of elif branches - deepens the tree by one level per link, so a chain of 100,000 links raises RecursionError or MemoryError where 100,000 separate statements compile. Code that generates source should emit flat statements or a lookup table rather than one long chain.

try:
    compile("x = " + "(" * 201 + "1" + ")" * 201, "<string>", "exec")
except SyntaxError as error:
    assert "too many nested parentheses" in str(error)
else:
    raise AssertionError("201 nested brackets compiled")

# Flat: one statement per case
flat = "".join(f"if a == {i}: b = {i}\n" for i in range(10_000))
compile(flat, "<string>", "exec")  # O(n)

Errors

compile() rejects invalid syntax, including scoping errors such as a nonlocal with no binding, but not a name that does not exist: that only fails when the code runs. A SyntaxError names the file and line it was given, which is why a meaningful filename helps.

try:
    compile("total = ", "settings.py", "exec")
except SyntaxError as error:
    assert error.filename == "settings.py"
    assert error.lineno == 1
else:
    raise AssertionError("incomplete source compiled")

code = compile("undefined_function()", "<string>", "exec")  # compiles
try:
    exec(code, {})
except NameError as error:
    assert "undefined_function" in str(error)
else:
    raise AssertionError("an undefined name ran")

Common Patterns

Loading a Configuration Script

config_script = """
DEBUG = True
DATABASE = "sqlite:///app.db"
ALLOWED_HOSTS = ["localhost", "127.0.0.1"]
"""

code = compile(config_script, "config.py", "exec")  # O(n), once
config = {}
exec(code, config)
assert config["DEBUG"] is True
assert config["ALLOWED_HOSTS"] == ["localhost", "127.0.0.1"]

Generating a Function

func_source = """
def fibonacci(n):
    a, b = 0, 1
    for _ in range(n):
        a, b = b, a + b
    return a
"""

namespace = {}
exec(compile(func_source, "<generated>", "exec"), namespace)  # O(n), once
fibonacci = namespace["fibonacci"]
assert [fibonacci(i) for i in range(8)] == [0, 1, 1, 2, 3, 5, 8, 13]

Performance Best Practices

✅ Do:

• Compile source that runs repeatedly once, and pass the code object to eval() or exec()
• Keep the code object rather than the string; compile() caches nothing
• Pass a real filename, so a SyntaxError points at the right place
• Generate flat statements, not one long elif or + chain

❌ Avoid:

• Calling compile() before a single eval() or exec() - they compile a string anyway, at the same cost
• Treating compile() as validation of untrusted code: it rejects only invalid syntax, and running the result runs whatever the code does
• Relying on assert in code compiled with optimize=1 or more, or under python -O

Related Functions

• eval() - Runs an expression, compiling a string on every call
• exec() - Runs statements, compiling a string on every call
• ast - Parse without compiling, and transform the tree before compiling it
• dis - Disassemble the code object compile() returns
• __future__ - The feature flags compile() accepts
• py_compile - Compile a file to a .pyc