py_compile — Compile Python source files

Source code: Lib/

The py_compile module provides a function to generate a byte-code file from a source file, and another function used when the module source file is invoked as a script.

Though not often needed, this function can be useful when installing modules for shared use, especially if some of the users may not have permission to write the byte-code cache files in the directory containing the source code.

exception py_compile.PyCompileError

Exception raised when an error occurs while attempting to compile the file.

py_compile.compile(file, cfile=None, dfile=None, doraise=False, optimize=- 1, invalidation_mode=PycInvalidationMode.TIMESTAMP, quiet=0)

Compile a source file to byte-code and write out the byte-code cache file. The source code is loaded from the file named file. The byte-code is written to cfile, which defaults to the PEP 3147/PEP 488 path, ending in .pyc. For example, if file is /foo/bar/ cfile will default to /foo/bar/__pycache__/baz.cpython-32.pyc for Python 3.2. If dfile is specified, it is used instead of file as the name of the source file from which source lines are obtained for display in exception tracebacks. If doraise is true, a PyCompileError is raised when an error is encountered while compiling file. If doraise is false (the default), an error string is written to sys.stderr, but no exception is raised. This function returns the path to byte-compiled file, i.e. whatever cfile value was used.

The doraise and quiet arguments determine how errors are handled while compiling file. If quiet is 0 or 1, and doraise is false, the default behaviour is enabled: an error string is written to sys.stderr, and the function returns None instead of a path. If doraise is true, a PyCompileError is raised instead. However if quiet is 2, no message is written, and doraise has no effect.

If the path that cfile becomes (either explicitly specified or computed) is a symlink or non-regular file, FileExistsError will be raised. This is to act as a warning that import will turn those paths into regular files if it is allowed to write byte-compiled files to those paths. This is a side-effect of import using file renaming to place the final byte-compiled file into place to prevent concurrent file writing issues.

optimize controls the optimization level and is passed to the built-in compile() function. The default of -1 selects the optimization level of the current interpreter.

invalidation_mode should be a member of the PycInvalidationMode enum and controls how the generated bytecode cache is invalidated at runtime. The default is PycInvalidationMode.CHECKED_HASH if the SOURCE_DATE_EPOCH environment variable is set, otherwise the default is PycInvalidationMode.TIMESTAMP.

Changed in version 3.2: Changed default value of cfile to be PEP 3147-compliant. Previous default was file + 'c' ('o' if optimization was enabled). Also added the optimize parameter.

Changed in version 3.4: Changed code to use importlib for the byte-code cache file writing. This means file creation/writing semantics now match what importlib does, e.g. permissions, write-and-move semantics, etc. Also added the caveat that FileExistsError is raised if cfile is a symlink or non-regular file.

Changed in version 3.7: The invalidation_mode parameter was added as specified in PEP 552. If the SOURCE_DATE_EPOCH environment variable is set, invalidation_mode will be forced to PycInvalidationMode.CHECKED_HASH.

Changed in version 3.7.2: The SOURCE_DATE_EPOCH environment variable no longer overrides the value of the invalidation_mode argument, and determines its default value instead.

Changed in version 3.8: The quiet parameter was added.

class py_compile.PycInvalidationMode

A enumeration of possible methods the interpreter can use to determine whether a bytecode file is up to date with a source file. The .pyc file indicates the desired invalidation mode in its header. See Cached bytecode invalidation for more information on how Python invalidates .pyc files at runtime.

New in version 3.7.


The .pyc file includes the timestamp and size of the source file, which Python will compare against the metadata of the source file at runtime to determine if the .pyc file needs to be regenerated.


The .pyc file includes a hash of the source file content, which Python will compare against the source at runtime to determine if the .pyc file needs to be regenerated.


Like CHECKED_HASH, the .pyc file includes a hash of the source file content. However, Python will at runtime assume the .pyc file is up to date and not validate the .pyc against the source file at all.

This option is useful when the .pycs are kept up to date by some system external to Python like a build system.

Command-Line Interface

This module can be invoked as a script to compile several source files. The files named in filenames are compiled and the resulting bytecode is cached in the normal manner. This program does not search a directory structure to locate source files; it only compiles files named explicitly. The exit status is nonzero if one of the files could not be compiled.

<file> ... <fileN>

Positional arguments are files to compile. If - is the only parameter, the list of files is taken from standard input.

-q, --quiet

Suppress errors output.

Changed in version 3.2: Added support for -.

Changed in version 3.10: Added support for -q.

See also

Module compileall

Utilities to compile all Python source files in a directory tree.