Collapses Python packages into a single module.
A package-based, source code amalgamater for collapsing Python packages into a single module.
The amalgamate utility enables the possibility of speeding up startup time for Python packages at low-to-no run time cost. If this sounds too good to be true, read on for the details!
The big idea here is to glue most of the source files in a package or subpackage together into a single module, called __amalgam__.py. Combined with some hooks in the __init__.py, this should dramatically reduce the number of files that are being searched for inside of the package. This is critical in larger projects where import times are the major startup time cost.
The amalgamate.py script automatically creates the __amalgamate__.py file for a package. This will create and solve a dependency graph of all modules in the package. It then will go through each module and glue it into the __amalgamate__.py, removing duplicate imports as it goes.
Additionally, the amalgamate.py script will automatically make most non-package imports lazy. This lets developers write as many imports as they want, without having to worry about startup times. Non-package modules aren’t actually imported in __amalgam__.py until they are used (ie an attribute is accessed).
This has some funny side effects such as,
- All of the modules that are amalgamated share the same globals (so be careful about naming things),
- Debugging makes most things looks like code comes from __amalgam__, unless an environment variable is set prior to import.
- Not all imports are able to be lazy.
The way the code amalgamater works is that other modules that are in the same package (and amalgamated) should be imported from-imports, without an as-clause. For example, suppose that z is a module in the package pkg, that depends on x and y in the same package. z should exclusively use imports like the following:
from pkg.x import a, c, d from pkg.y import e, f, g
These from-imports simulate all of the x, y, and z modules having the same globals(). This is because the amalgamater puts all such modules in the same globals(), which is effectively what the from-imports do. For example, xonsh.ast and xonsh.execer are both in the same package (xonsh). Thus they should use the above from from-import syntax.
Alternatively, for modules outside of the current package (or modules that are not amalgamated) the import statement should be either import pkg.x or import pkg.x as name. This is because these are the only cases where the amalgamater is able to automatically insert lazy imports in way that is guaranteed to be safe. Say we are back in z and depend on dep, collections.abc, and modules in a subpackage, pkg.sub. The following are all acceptable:
import dep import collections.abc as abc import pkg.dep.mod0 import pkg.dep.mod1 as mod1
The important thing here is to simply be consistent for such imports across all modules in the package pkg.
WARNING: You should not use the form from pkg.i import j for modules outside of the amalgamated package. This is due to the ambiguity that from pkg.x import name may import a variable that cannot be lazily constructed OR may import a module. The amalgamater is forced to leave such import statements as they were written, which means that they cannot be automatically lazy or eliminated. They are thus forced to be imported at when __amalgam__.py is imported/
So the simple rules to follow are that:
- Import objects from modules in the same package directly in using from-import,
- Import objects from modules outside of the package via a direct import or import-as statement.
To make this all work, the __init__.py for the package needs a predefined space for amalgamate.py to write hooks into. In its simplest form, this is defined by the lines:
# amalgamate exclude # amalgamate end
The amalgamate.py script will fill in between these two line and will over write them as needed. The initial exclude line accepts a space-separated list of module names in the package to exclude from amalgamation:
# amalgamate exclude foo bar baz # amalgamate end
You may also provide as many exclude lines as you want, though there should only be one end line:
# amalgamate exclude foo # amalgamate exclude bar # amalgamate exclude baz # amalgamate end
Also note that all modules whose names start with a double underscore, like __init__.py and __main__.py are automatically excluded.
Command Line Interface
The command line interface is a list of package names to amalgamate:
$ amalgamate.py pkg pkg.sub0 pkg.sub1
You may also provide the --debug=NAME name to declare the environment variable name for import debugging:
$ amalgamate.py --debug=PKG_DEBUG pkg pkg.sub0 pkg.sub1
By default, this environment variable is simply called DEBUG. If this environment variable exists and is set to a non-empty string, then all amalgamated imports are skipped and the modules in the package are imported normally. For example, suppose you have a script that imports your package and you would like to see the module names, you could run the script with:
$ env PKG_DEBUG=1 python script.py
to suppress the amalgamated imports.
We recommend running amalgamate.py every time that setup.py is executed. This keeps __amalgam__.py and __init__.py in sync with the rest of the package. Feel free to use the following hook function in your project:
def amalgamate_source(): """Amalgamates source files.""" try: import amalgamate except ImportError: print('Could not import amalgamate, skipping.', file=sys.stderr) return amalgamate.main(['amalgamate', '--debug=PKG_DEBUG', 'pkg'])
Additionally, feel free to copy the amalgamate.py script to your project. It is only a single file!
This is implemented via a syntax tree transformation so developers could write mostly normal Python without having to worry about import speed. That accounts for the wizardry.
The darkness comes from a project called JsonCpp. JsonCpp has an amalgamate script, that glues the whole project into a single header and single source file. This is an amazing idea. The kicker is that JsonCpp’s amalgamate is written in Python :)
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