README.rst

lazyasd

A package that provides lazy and self-destructive tools for speeding up module imports. This is useful whenever startup times are critical, such as for command line interfaces or other user-facing applications.

The tools in this module implement two distinct strategies for speeding up module import. The first is delayed construction of global state and the second is to import expensive modules in a background thread.

Feel free to use lazyasd as a dependency or, because it is implemented as a single module, copy the lazyasd.py file into your project.

Lazy Construction

Many operations related to data construction or inspection setup can take a long time to complete. If only a single copy of the data or a cached representation is needed, in Python it is common to move the data to the global or module level scope.

By moving to module level, we help ensure that only a single copy of the data is ever built. However, by moving to module scope, the single performance hit now comes at import time. This is itself wasteful if the data is never used. Furthermore, the more data that is built globally, the longer importing the module takes.

For example, consider a function that reports if a string contains the word "foo" using regular expressions. The naive version is relatively slow, per function call, because it has to construct the regex each time:

import re

def has_foo_simple(s):
    return re.search('foo', s) is not None

The standard way of improving performance is to compile the regex at global scope. Rewriting, we would see:

import re

FOO_RE = re.compile('foo')

def has_foo_compiled(s):
    return FOO_RE.search(s) is not None

Now, each call of has_foo_compiled() is much faster than a call of has_foo_simple() because we have shifted the compilation to import time. But what if we never actually call has_foo()? In this case, the original version was better because the imports are fast.

Having the best of both compile-once and don't-compile-on-import is where the lazy and self-destructive tools come in. A LazyObject instance has a loader function, a context to place the result of the into, and the name of the loaded value in the context. The LazyObject does no work when it is first created. However, whenever an attribute is accessed (or a variety of other operations) the loader will be called, the true value will be constructed, and the LazyObject will act as a proxy to loaded object.

Using the above regex example, we have minimal import-time and run-time performance hits with the following lazy implementation:

import re
from lazyasd import LazyObject

FOO_RE = LazyObject(lambda: re.compile('foo'), globals(), 'FOO_RE')

def has_foo_lazy(s):
    return FOO_RE.search(s) is not None

To walk through the above, at import time FOO_RE is a LazyObject, that has a lambda loader which returns the regex we care about. If FOO_RE is never accessed this is how it will remain. However, the first time has_foo_lazy() is called, accessing the search method will cause the LazyObject to:

1. Call the loader (getting re.compile('foo') as the result)
2. Place the result in the context, eg globals()['FOO_RE'] = re.compile('foo')
3. Look up attributes and methods (such as search) on the result.

Now because of the context replacement, FOO_RE now is a regular expression object. Further calls to has_foo_lazy() will see FOO_RE as a regular expression object directly, and not as a LazyObject. In fact, if no lingering references remain, the original LazyObject instance can be totally cleaned up by the garbage collector!

For the truly lazy, there is also a lazyobject decorator:

import re
from lazyasd import lazyobject

@lazyobject
def foo_re():
    return re.compile('foo')

def has_foo_lazy(s):
    return foo_re.search(s) is not None

Another useful pattern is to implement lazy module imports, where the module is only imported if a member of it used:

import importlib
from lazyasd import lazyobject

@lazyobject
def os():
    return importlib.import_module('os')

The world is beautifully yours, but feel free to take a nap first.

Specific Laziness

The LazyBool class and lazybool decorator have the same interface as lazy objects. These are provided for objects that are intended to be resolved as booleans.

The LazyDict class and lazydict decorator are similar. Here however, the first value is a dictionary of key-loaders. Rather than having a single loader, each value is loaded individually when its key is first accessed.

Background Imports

Even with all of the above laziness, sometimes it isn't enough. Sometimes a module is so painful to import and so unavoidable that you need to import it on background thread so that the rest of the application can boot up in the meantime. This is the purpose of load_module_in_background().

For example, if you are using pygments and you want the import to safely be 100x faster, simply drop in the following lines:

# must come before pygments imports
from lazyasd import load_module_in_background
load_module_in_background('pkg_resources',
                          replacements={'pygments.plugin': 'pkg_resources'})

# now pygments is fast to import
from pygments.style import Style

This prevents pkg_resources, which comes from setuptools, from searching your entire filesystem for plugins at import time. Like above, this import acts as proxy and will block until it is needed. It is also robust if the module has already been imported. In some cases, this background importing is the best a third party application can do.