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getattr() Function Complexity

The getattr() function retrieves the value of a named attribute from an object: getattr(obj, 'name') does the same lookup as obj.name, with the name supplied at run time. It costs what that lookup costs: a cached class-level lookup plus an instance dict probe, or whatever a user-defined hook does.

Complexity Reference

Let d be the length of the object's class MRO and g the cost, in time and space, of a user-defined hook: __getattr__, __getattribute__, or a descriptor such as a property getter.

Operation Time Space Notes
getattr(obj, name) O(1) avg O(1) The class-level lookup is cached per class and name object, for misses as well as hits, so d does not matter once the cache is warm - provided name is the same str object each time, as a literal in the source or a sys.intern() result is. A miss raises AttributeError
getattr(obj, name) with a name built at run time O(d) O(1) An f-string, a concatenation or a string read from input is usually a new object on every call, and the cache matches name objects rather than their text, so every call walks the MRO. sys.intern(name) returns one shared object per text and restores the cached cost. A str subclass, or a name longer than 100 characters, is never cached
getattr(obj, name, default) O(1) avg O(1) The same lookup; default is returned only when the lookup raises AttributeError. Any other exception propagates. The default expression is evaluated before the call, whether or not it is used
getattr(obj, name), first lookup after the class or a base changed O(d) O(1) Walks the MRO once and caches the result. From Python 3.13, once a class has been changed and looked up again more than 1,000 times it is never cached again, and every lookup on it pays O(d)
getattr(obj, name) through a hook O(g) O(g) On top of the lookup above. A property getter runs on every call, and __getattr__ runs on every miss
getattr(obj, name) with a non-str name O(1) O(1) Raises TypeError before any lookup, with or without a default

Basic Usage

Dynamic Attribute Names

# O(1) avg per name - the same lookup as obj.name
class Config:
    host = "localhost"
    port = 8000
    debug = True

config = Config()

assert getattr(config, 'host') == config.host  # O(1) avg

values = {name: getattr(config, name) for name in ['host', 'port', 'debug']}
assert values == {'host': 'localhost', 'port': 8000, 'debug': True}

Get with Default Value

# O(1) avg - the default replaces an AttributeError, nothing else
class User:
    name = "Alice"
    email = None

user = User()

phone = getattr(user, 'phone', 'Not provided')  # O(1) avg - a miss
email = getattr(user, 'email', 'no@email.com')  # O(1) avg - a hit

assert phone == 'Not provided'
assert email is None  # the attribute exists, so its value wins over the default

try:
    getattr(user, 'phone')
except AttributeError as error:
    assert "'phone'" in str(error)
else:
    raise AssertionError("a miss without a default should raise")

Lookup Cost

Inheritance Chain Traversal

# The first lookup of a name on a class walks the MRO - O(d)
# Later lookups of that name on that class are cached - O(1) avg
class A:
    attr_a = 'A'

class B(A):
    attr_b = 'B'

class C(B):
    attr_c = 'C'

class D(C):
    attr_d = 'D'

obj = D()
assert D.__mro__ == (D, C, B, A, object)

val = getattr(obj, 'attr_a')  # O(d) once: D, C, B, then found in A
assert val == 'A'
val = getattr(obj, 'attr_a')  # O(1) avg: cached for (D, 'attr_a')
assert val == 'A'

missing = getattr(obj, 'attr_x', None)  # O(d) once, then the miss is cached too
assert missing is None

# Assigning to a class, or to any of its bases, invalidates its cache
C.attr_c = 'C2'
val = getattr(obj, 'attr_a')  # O(d) again, once
assert val == 'A' and getattr(obj, 'attr_c') == 'C2'

What the Default Catches

# Only AttributeError becomes the default; every other exception propagates
class Strict:
    @property
    def broken(self):
        raise KeyError("not an AttributeError")

    @property
    def absent(self):
        raise AttributeError("absent")

obj = Strict()

assert getattr(obj, 'missing', None) is None
assert getattr(obj, 'absent', None) is None  # the getter's AttributeError counts

try:
    getattr(obj, 'broken', None)
except KeyError:
    pass
else:
    raise AssertionError("the KeyError should propagate")

try:
    getattr(obj, 42, None)
except TypeError as error:
    assert "must be string" in str(error)
else:
    raise AssertionError("a non-str name should raise even with a default")

The Default Is Evaluated Eagerly

# The default is an ordinary argument: it is built before getattr() runs
calls = 0

def expensive_default():
    global calls
    calls += 1
    return []

class Cache:
    items = [1, 2, 3]

cache = Cache()

items = getattr(cache, 'items', expensive_default())  # pays for the default anyway
assert items == [1, 2, 3] and calls == 1

# Build the default only on a miss - a private sentinel, since None may be a real value
_missing = object()
items = getattr(cache, 'items', _missing)
if items is _missing:
    items = expensive_default()
assert items == [1, 2, 3] and calls == 1

orders = getattr(cache, 'orders', _missing)
if orders is _missing:
    orders = expensive_default()
assert orders == [] and calls == 2

Performance Patterns

getattr vs hasattr

# getattr() with a default - one lookup
class Settings:
    timeout = 30

obj = Settings()

value = getattr(obj, 'timeout', None)  # O(1) avg - one lookup
assert value == 30

# hasattr() then getattr() - two lookups of the same name
if hasattr(obj, 'timeout'):
    value = getattr(obj, 'timeout')
else:
    value = None

assert value == 30

Direct Access vs getattr

# Both do the same O(1) avg lookup; getattr() adds one builtin call
class Data:
    value = 42

obj = Data()

direct = obj.value  # name fixed in the source
dynamic = getattr(obj, 'value')  # name chosen at run time
assert direct == dynamic == 42

# With a name known in advance, write obj.value; getattr() is for names held in variables

Common Patterns

Dispatch by Name

# O(1) avg - one lookup picks the handler, once the built name is interned
import sys

class Processor:
    def process_int(self, val):
        return val * 2

    def process_str(self, val):
        return val.upper()

    def process_list(self, val):
        return len(val)

    def process(self, val):
        """Dispatch on the value's type name - one lookup"""
        # The f-string is a new object per call; interned, it hits the lookup cache
        name = sys.intern(f"process_{type(val).__name__}")
        method = getattr(self, name, None)
        if method is None:
            return None
        return method(val)

processor = Processor()

assert processor.process(5) == 10
assert processor.process("hello") == "HELLO"
assert processor.process([1, 2, 3]) == 3
assert processor.process(1.5) is None

Optional Hooks

# O(1) avg per event - call a handler only if the object defines one
class Plugin:
    def __init__(self):
        self.calls = []

    def on_init(self):
        self.calls.append('init')

    def on_start(self):
        self.calls.append('start')

def trigger(obj, event_name):
    """Call an event handler if it exists - one lookup"""
    handler = getattr(obj, event_name, None)
    if callable(handler):
        handler()

plugin = Plugin()

trigger(plugin, 'on_init')
trigger(plugin, 'on_start')
trigger(plugin, 'on_stop')  # no handler, no error
assert plugin.calls == ['init', 'start']

Reading Several Names

# O(n) - one lookup per name, for n names
class Settings:
    debug = False
    timeout = 30
    max_connections = 100

def snapshot(obj, names):
    """Read the named attributes into a dict - O(n)"""
    return {name: getattr(obj, name) for name in names}

assert snapshot(Settings(), ['debug', 'timeout']) == {'debug': False, 'timeout': 30}

# dir() lists and sorts every attribute of the object and its whole MRO,
# so copying "everything" grows with that count, not with the names wanted
public = [name for name in dir(Settings()) if not name.startswith('_')]
assert public == ['debug', 'max_connections', 'timeout']

Edge Cases

Hooks Run Inside getattr()

# A property getter runs on every getattr(); __getattr__ runs on every miss
class Example:
    present = 1

    def __init__(self):
        self.getter_runs = 0
        self.misses = []

    @property
    def computed(self):  # its cost is the call's
        self.getter_runs += 1
        return 100

    def __getattr__(self, name):  # reached only when the normal lookup fails
        self.misses.append(name)
        raise AttributeError(name)

obj = Example()

assert getattr(obj, 'computed') == 100
assert getattr(obj, 'computed') == 100
assert obj.getter_runs == 2

assert getattr(obj, 'present') == 1
assert obj.misses == []
assert getattr(obj, 'missing', None) is None
assert obj.misses == ['missing']

getattribute Sees Every Lookup

# __getattribute__ replaces the lookup itself - every getattr() call runs it
lookups = 0

class Counting:
    attr = 1

    def __getattribute__(self, name):
        global lookups
        lookups += 1
        return super().__getattribute__(name)

obj = Counting()

getattr(obj, 'attr')  # one lookup
getattr(obj, 'missing', None)  # one lookup, then the default
assert lookups == 2

Performance Best Practices

✅ Do:

  • Use getattr() with a default instead of hasattr() followed by getattr() - one lookup, not two
  • Pass a private sentinel as the default and build the real fallback only when it comes back, when the fallback is expensive
  • Write obj.name when the name is fixed in the source
  • Pass a name built at run time through sys.intern() when it is looked up repeatedly on a deep hierarchy - a fresh string object misses the lookup cache
  • Expect property getters and __getattr__ to run inside it

❌ Avoid:

  • An expensive expression as the default - it runs even when the attribute exists
  • Relying on the default to swallow anything but AttributeError
  • dir() as a way to read a few known names - it builds and sorts the whole attribute list
  • Reassigning a class's attributes in a hot loop on Python 3.13 or later - after more than 1,000 rounds of change and lookup the class is never cached again

Version Notes

  • Python 3.13+: A class that has been changed and looked up again more than 1,000 times stops being cached, so lookups on it are O(d) from then on
  • All Python 3: A non-str name raises TypeError, with or without a default