Notebook

Module Design¶

Minimize Coupling: Avoid Global Variables
Maximize Cohesion: One Purpose

Data Hiding¶

There is no way to prevent a client from changing names inside a module
In Python, data hiding is a convention, not a constraint.

`_x` and `all`¶

Name with a single underscore (_x)
- What not to be copied
- A client imports using from * will ignore these variables
- It's not private declaration. You can still import them.
__all__
- What to be copied, higher priority that _x
- When this feature is used, from * will copy out only names listed in __all__ list.

Enabling Future Language Features `future`¶

from __future__ import featurename

In [1]:

import __future__

[
    featurename
    for featurename in dir(__future__)
    if not featurename.startswith("_") and featurename[0].islower()
]

Out[1]:

['absolute_import',
 'all_feature_names',
 'barry_as_FLUFL',
 'division',
 'generators',
 'nested_scopes',
 'print_function',
 'unicode_literals',
 'with_statement']

Although import from future is supported, there is no way to import from the past

Mixed Usage Modes: `name` and `main`¶

If the file is being run as a top-level program, __name__ is set to __main__.
If it's imported, __name__ is set to the modules's name as known by its clients

This __name__ servers as usage mode flag

In [2]:

if __name__ == "__main__":
    print(__name__)

__main__

The as Extension for import and from¶

import as¶

import modulename as name

is the same as

import modulename
name = modulename
del modulename

from import as¶

from module1 import utility as util1

Import Module by Name String¶

Original import cannot import a module by name string

The following two way can do so

__import__
- It's is generally intended for customizing import operations by reassignment in the built-in scope
importlib.import_module
- The optional second argument can give the package the anchor point for resolving relative imports

They returns the module object, so assign it to a name to keep it

In [3]:

module_name = "os"
os = __import__(module_name)
dir(os)

Out[3]:

['F_OK',
 'MutableMapping',
 'O_APPEND',
 'O_BINARY',
 'O_CREAT',
 'O_EXCL',
 'O_NOINHERIT',
 'O_RANDOM',
 'O_RDONLY',
 'O_RDWR',
 'O_SEQUENTIAL',
 'O_SHORT_LIVED',
 'O_TEMPORARY',
 'O_TEXT',
 'O_TRUNC',
 'O_WRONLY',
 'P_DETACH',
 'P_NOWAIT',
 'P_NOWAITO',
 'P_OVERLAY',
 'P_WAIT',
 'R_OK',
 'SEEK_CUR',
 'SEEK_END',
 'SEEK_SET',
 'TMP_MAX',
 'W_OK',
 'X_OK',
 '_Environ',
 '__all__',
 '__builtins__',
 '__cached__',
 '__doc__',
 '__file__',
 '__loader__',
 '__name__',
 '__package__',
 '__spec__',
 '_execvpe',
 '_exists',
 '_exit',
 '_get_exports_list',
 '_putenv',
 '_unsetenv',
 '_wrap_close',
 'abort',
 'access',
 'altsep',
 'chdir',
 'chmod',
 'close',
 'closerange',
 'cpu_count',
 'curdir',
 'defpath',
 'device_encoding',
 'devnull',
 'dup',
 'dup2',
 'environ',
 'errno',
 'error',
 'execl',
 'execle',
 'execlp',
 'execlpe',
 'execv',
 'execve',
 'execvp',
 'execvpe',
 'extsep',
 'fdopen',
 'fsdecode',
 'fsencode',
 'fstat',
 'fsync',
 'get_exec_path',
 'get_handle_inheritable',
 'get_inheritable',
 'get_terminal_size',
 'getcwd',
 'getcwdb',
 'getenv',
 'getlogin',
 'getpid',
 'getppid',
 'isatty',
 'kill',
 'linesep',
 'link',
 'listdir',
 'lseek',
 'lstat',
 'makedirs',
 'mkdir',
 'name',
 'open',
 'pardir',
 'path',
 'pathsep',
 'pipe',
 'popen',
 'putenv',
 'read',
 'readlink',
 'remove',
 'removedirs',
 'rename',
 'renames',
 'replace',
 'rmdir',
 'sep',
 'set_handle_inheritable',
 'set_inheritable',
 'spawnl',
 'spawnle',
 'spawnv',
 'spawnve',
 'st',
 'startfile',
 'stat',
 'stat_float_times',
 'stat_result',
 'statvfs_result',
 'strerror',
 'supports_bytes_environ',
 'supports_dir_fd',
 'supports_effective_ids',
 'supports_fd',
 'supports_follow_symlinks',
 'symlink',
 'sys',
 'system',
 'terminal_size',
 'times',
 'times_result',
 'umask',
 'uname_result',
 'unlink',
 'urandom',
 'utime',
 'waitpid',
 'walk',
 'write']

In [4]:

import importlib

module_name = "sys"
sys = importlib.import_module(module_name)
dir(sys)

Out[4]:

['__displayhook__',
 '__doc__',
 '__excepthook__',
 '__interactivehook__',
 '__loader__',
 '__name__',
 '__package__',
 '__spec__',
 '__stderr__',
 '__stdin__',
 '__stdout__',
 '_clear_type_cache',
 '_current_frames',
 '_debugmallocstats',
 '_getframe',
 '_home',
 '_mercurial',
 '_xoptions',
 'api_version',
 'argv',
 'base_exec_prefix',
 'base_prefix',
 'builtin_module_names',
 'byteorder',
 'call_tracing',
 'callstats',
 'copyright',
 'displayhook',
 'dllhandle',
 'dont_write_bytecode',
 'exc_info',
 'excepthook',
 'exec_prefix',
 'executable',
 'exit',
 'flags',
 'float_info',
 'float_repr_style',
 'getallocatedblocks',
 'getcheckinterval',
 'getdefaultencoding',
 'getfilesystemencoding',
 'getprofile',
 'getrecursionlimit',
 'getrefcount',
 'getsizeof',
 'getswitchinterval',
 'gettrace',
 'getwindowsversion',
 'hash_info',
 'hexversion',
 'implementation',
 'int_info',
 'intern',
 'maxsize',
 'maxunicode',
 'meta_path',
 'modules',
 'path',
 'path_hooks',
 'path_importer_cache',
 'platform',
 'prefix',
 'ps1',
 'ps2',
 'ps3',
 'setcheckinterval',
 'setprofile',
 'setrecursionlimit',
 'setswitchinterval',
 'settrace',
 'stderr',
 'stdin',
 'stdout',
 'thread_info',
 'version',
 'version_info',
 'warnoptions',
 'winver']

Transitive Module Reloads¶

To achieve it, scanning modules' __dict__ namespace and check each item's type to find nested modules to reload.
It works only on import, not from.
The transitive reloader relies on the fact that module reloads update module objects in place.

Module Gotchas¶

Statement Order Matters in Top-Level Code¶

Don't mix def with top-level code.
Put def at the top of the file and top-level code at bottom

In [5]:

def func():
    print(x)


x = 1

When imported, the above module would cause an error.

from Copies Names but Doesn't Link¶

from is a name-copy operation, not a name aliasing.

`from *` Can Obscure the Meaning of Variables¶

Again, avoiding using from * or at most once per file.