Notebook

import and from¶

Both import and from are statements, not compile-time declarations.
Thus, they can be nested in if like other statements.

import¶

Assigning an entire module object to a single name

from¶

Copy specific names from one file to another.
Assigning one or more names to objects of the same names in another module.
- No link from a name copied with from back to the original file.
- To really change a global name in another file, you must use import.(Not Recommaned design)

from gotchas¶

from only copies names from one module to another, it does not assign the module name itself
from always load the whole file into memory.
There is no way to load part of a module file.

from ... import *¶

Get copies of all names assigned at the top level of referenced module.
Note that only * works. Other pattern matching rule won't work.

In Python3, from ... import * can only be used at the top level of a module file, not within functions.
In Python2, it's allowed, but will issue a warning

Using `from ... import ` statement might hide variables that have the same names without notice*
Thus, the following design is recommanded.

from moduel import x, y, z

import and from Equivalence¶

from¶

from module import name1, name2

import¶

import module
name1 = module.name1
name2 = module.name2

import or from?¶

Prefer import to from for simple modules
Explicityly list the variables you want in from statements
- Limit from module import * to at most once per file

When import is required?¶

When you must use the same name defined in two different modules

e.g.

# a.py and b.py exist in the same folder to be used as a sample

# a.py
def func():
    print("This is a")

# b.py
def func():
    print("This is b")

In [1]:

# main.py
from a import func
from b import func

func()

This is b

In [2]:

# main2.py
import a, b

a.func()
b.func()

This is a
This is b

Another way to solve this dilemma is using as statement

In [3]:

from a import func as afunc
from b import func as bfunc

afunc()
bfunc()

This is a
This is b

Module Namespace¶

Module namespaces can be accessed via the attribute __dict__ or dir(M)

In [4]:

import os

for key, value in os.__dict__.items():
    print(key)
    print("\t", value)

SEEK_END
	 2
pardir
	 ..
execl
	 <function execl at 0x01732A98>
execve
	 <built-in function execve>
curdir
	 .
link
	 <built-in function link>
_execvpe
	 <function _execvpe at 0x01732C48>
sys
	 <module 'sys' (built-in)>
access
	 <built-in function access>
O_TEXT
	 16384
chdir
	 <built-in function chdir>
urandom
	 <built-in function urandom>
_wrap_close
	 <class 'os._wrap_close'>
devnull
	 nul
W_OK
	 2
spawnl
	 <function spawnl at 0x01749A98>
P_NOWAITO
	 3
supports_fd
	 {<built-in function stat>}
O_SHORT_LIVED
	 4096
O_BINARY
	 32768
pathsep
	 ;
O_RDWR
	 2
SEEK_CUR
	 1
execlp
	 <function execlp at 0x01732B28>
lstat
	 <built-in function lstat>
dup2
	 <built-in function dup2>
popen
	 <function popen at 0x01749BB8>
read
	 <built-in function read>
set_inheritable
	 <built-in function set_inheritable>
defpath
	 .;C:\bin
__builtins__
	 {'bool': <class 'bool'>, 'EOFError': <class 'EOFError'>, 'sum': <built-in function sum>, 'enumerate': <class 'enumerate'>, 'dict': <class 'dict'>, 'Exception': <class 'Exception'>, 'ascii': <built-in function ascii>, 'map': <class 'map'>, 'DeprecationWarning': <class 'DeprecationWarning'>, 'ord': <built-in function ord>, 'IsADirectoryError': <class 'IsADirectoryError'>, 'BytesWarning': <class 'BytesWarning'>, 'list': <class 'list'>, 'exec': <built-in function exec>, 'OverflowError': <class 'OverflowError'>, 'float': <class 'float'>, 'id': <built-in function id>, 'locals': <built-in function locals>, 'BlockingIOError': <class 'BlockingIOError'>, 'FileExistsError': <class 'FileExistsError'>, 'SystemExit': <class 'SystemExit'>, 'tuple': <class 'tuple'>, 'BaseException': <class 'BaseException'>, 'hasattr': <built-in function hasattr>, 'AttributeError': <class 'AttributeError'>, '__IPYTHON__active': 'Deprecated, check for __IPYTHON__', 'all': <built-in function all>, 'max': <built-in function max>, 'InterruptedError': <class 'InterruptedError'>, 'oct': <built-in function oct>, 'GeneratorExit': <class 'GeneratorExit'>, 'NotImplementedError': <class 'NotImplementedError'>, 'NotADirectoryError': <class 'NotADirectoryError'>, 'set': <class 'set'>, 'eval': <built-in function eval>, 'NameError': <class 'NameError'>, 'ConnectionResetError': <class 'ConnectionResetError'>, 'EnvironmentError': <class 'OSError'>, 'Ellipsis': Ellipsis, 'repr': <built-in function repr>, 'IOError': <class 'OSError'>, 'help': Type help() for interactive help, or help(object) for help about object., 'staticmethod': <class 'staticmethod'>, 'FloatingPointError': <class 'FloatingPointError'>, 'FutureWarning': <class 'FutureWarning'>, 'BrokenPipeError': <class 'BrokenPipeError'>, 'IndexError': <class 'IndexError'>, 'SystemError': <class 'SystemError'>, 'ConnectionRefusedError': <class 'ConnectionRefusedError'>, 'Warning': <class 'Warning'>, 'RuntimeWarning': <class 'RuntimeWarning'>, 'BufferError': <class 'BufferError'>, 'slice': <class 'slice'>, 'license': See https://www.python.org/psf/license/, 'object': <class 'object'>, 'UnboundLocalError': <class 'UnboundLocalError'>, 'len': <built-in function len>, 'any': <built-in function any>, 'UnicodeEncodeError': <class 'UnicodeEncodeError'>, 'ResourceWarning': <class 'ResourceWarning'>, 'UserWarning': <class 'UserWarning'>, 'None': None, '__package__': '', 'reversed': <class 'reversed'>, 'frozenset': <class 'frozenset'>, 'FileNotFoundError': <class 'FileNotFoundError'>, 'dir': <built-in function dir>, 'callable': <built-in function callable>, 'hex': <built-in function hex>, '__doc__': "Built-in functions, exceptions, and other objects.\n\nNoteworthy: None is the `nil' object; Ellipsis represents `...' in slices.", 'type': <class 'type'>, 'setattr': <built-in function setattr>, 'abs': <built-in function abs>, 'zip': <class 'zip'>, 'SyntaxWarning': <class 'SyntaxWarning'>, '__name__': 'builtins', 'str': <class 'str'>, 'memoryview': <class 'memoryview'>, '__import__': <built-in function __import__>, 'compile': <built-in function compile>, 'vars': <built-in function vars>, 'SyntaxError': <class 'SyntaxError'>, 'open': <built-in function open>, 'property': <class 'property'>, 'NotImplemented': NotImplemented, 'AssertionError': <class 'AssertionError'>, 'min': <built-in function min>, 'ZeroDivisionError': <class 'ZeroDivisionError'>, 'int': <class 'int'>, 'isinstance': <built-in function isinstance>, 'ConnectionAbortedError': <class 'ConnectionAbortedError'>, 'format': <built-in function format>, 'globals': <built-in function globals>, 'hash': <built-in function hash>, 'copyright': Copyright (c) 2001-2015 Python Software Foundation.
All Rights Reserved.

Copyright (c) 2000 BeOpen.com.
All Rights Reserved.

Copyright (c) 1995-2001 Corporation for National Research Initiatives.
All Rights Reserved.

Copyright (c) 1991-1995 Stichting Mathematisch Centrum, Amsterdam.
All Rights Reserved., 'super': <class 'super'>, 'UnicodeDecodeError': <class 'UnicodeDecodeError'>, 'PermissionError': <class 'PermissionError'>, 'TimeoutError': <class 'TimeoutError'>, 'ConnectionError': <class 'ConnectionError'>, 'sorted': <built-in function sorted>, 'print': <built-in function print>, 'round': <built-in function round>, 'MemoryError': <class 'MemoryError'>, 'credits':     Thanks to CWI, CNRI, BeOpen.com, Zope Corporation and a cast of thousands
    for supporting Python development.  See www.python.org for more information., 'ChildProcessError': <class 'ChildProcessError'>, 'ProcessLookupError': <class 'ProcessLookupError'>, 'False': False, 'getattr': <built-in function getattr>, 'TabError': <class 'TabError'>, 'ArithmeticError': <class 'ArithmeticError'>, 'dreload': <function reload at 0x02B23AE0>, 'bin': <built-in function bin>, 'chr': <built-in function chr>, 'iter': <built-in function iter>, '__IPYTHON__': True, 'True': True, 'IndentationError': <class 'IndentationError'>, 'LookupError': <class 'LookupError'>, 'ImportWarning': <class 'ImportWarning'>, 'OSError': <class 'OSError'>, 'WindowsError': <class 'OSError'>, 'ImportError': <class 'ImportError'>, 'delattr': <built-in function delattr>, 'get_ipython': <bound method ZMQInteractiveShell.get_ipython of <IPython.kernel.zmq.zmqshell.ZMQInteractiveShell object at 0x02B0B390>>, '__build_class__': <built-in function __build_class__>, 'bytes': <class 'bytes'>, 'KeyError': <class 'KeyError'>, 'ValueError': <class 'ValueError'>, 'ReferenceError': <class 'ReferenceError'>, 'bytearray': <class 'bytearray'>, 'UnicodeWarning': <class 'UnicodeWarning'>, 'PendingDeprecationWarning': <class 'PendingDeprecationWarning'>, '__spec__': ModuleSpec(name='builtins', loader=<class '_frozen_importlib.BuiltinImporter'>), 'divmod': <built-in function divmod>, 'StopIteration': <class 'StopIteration'>, 'range': <class 'range'>, 'next': <built-in function next>, 'RuntimeError': <class 'RuntimeError'>, 'filter': <class 'filter'>, '__debug__': True, 'UnicodeTranslateError': <class 'UnicodeTranslateError'>, 'TypeError': <class 'TypeError'>, 'complex': <class 'complex'>, 'pow': <built-in function pow>, 'UnicodeError': <class 'UnicodeError'>, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, 'classmethod': <class 'classmethod'>, 'issubclass': <built-in function issubclass>, 'input': <bound method IPythonKernel.raw_input of <IPython.kernel.zmq.ipkernel.IPythonKernel object at 0x02B0B3B0>>, 'KeyboardInterrupt': <class 'KeyboardInterrupt'>}
rename
	 <built-in function rename>
device_encoding
	 <built-in function device_encoding>
P_WAIT
	 0
O_CREAT
	 256
spawnve
	 <built-in function spawnve>
linesep
	 

execv
	 <built-in function execv>
name
	 nt
spawnv
	 <built-in function spawnv>
listdir
	 <built-in function listdir>
get_inheritable
	 <built-in function get_inheritable>
__all__
	 ['altsep', 'curdir', 'pardir', 'sep', 'pathsep', 'linesep', 'defpath', 'name', 'path', 'devnull', 'SEEK_SET', 'SEEK_CUR', 'SEEK_END', 'fsencode', 'fsdecode', 'get_exec_path', 'fdopen', 'popen', 'extsep', '_exit', 'F_OK', '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', 'TMP_MAX', 'W_OK', 'X_OK', 'abort', 'access', 'chdir', 'chmod', 'close', 'closerange', 'cpu_count', 'device_encoding', 'dup', 'dup2', 'environ', 'error', 'execv', 'execve', 'fstat', 'fsync', 'get_handle_inheritable', 'get_inheritable', 'get_terminal_size', 'getcwd', 'getcwdb', 'getlogin', 'getpid', 'getppid', 'isatty', 'kill', 'link', 'listdir', 'lseek', 'lstat', 'mkdir', 'open', 'pipe', 'putenv', 'read', 'readlink', 'remove', 'rename', 'replace', 'rmdir', 'set_handle_inheritable', 'set_inheritable', 'spawnv', 'spawnve', 'startfile', 'stat', 'stat_float_times', 'stat_result', 'statvfs_result', 'strerror', 'symlink', 'system', 'terminal_size', 'times', 'times_result', 'umask', 'uname_result', 'unlink', 'urandom', 'utime', 'waitpid', 'write', 'makedirs', 'removedirs', 'renames', 'walk', 'execl', 'execle', 'execlp', 'execlpe', 'execvp', 'execvpe', 'getenv', 'supports_bytes_environ', 'spawnl', 'spawnle']
getppid
	 <built-in function getppid>
write
	 <built-in function write>
isatty
	 <built-in function isatty>
stat_float_times
	 <built-in function stat_float_times>
execvpe
	 <function execvpe at 0x01732C00>
get_exec_path
	 <function get_exec_path at 0x01732C90>
MutableMapping
	 <class 'collections.abc.MutableMapping'>
fstat
	 <built-in function fstat>
remove
	 <built-in function remove>
supports_effective_ids
	 set()
get_terminal_size
	 <built-in function get_terminal_size>
chmod
	 <built-in function chmod>
__package__
	 
O_RANDOM
	 16
R_OK
	 4
makedirs
	 <function makedirs at 0x01717B70>
X_OK
	 1
O_WRONLY
	 1
__doc__
	 OS routines for NT or Posix depending on what system we're on.

This exports:
  - all functions from posix, nt or ce, e.g. unlink, stat, etc.
  - os.path is either posixpath or ntpath
  - os.name is either 'posix', 'nt' or 'ce'.
  - os.curdir is a string representing the current directory ('.' or ':')
  - os.pardir is a string representing the parent directory ('..' or '::')
  - os.sep is the (or a most common) pathname separator ('/' or ':' or '\\')
  - os.extsep is the extension separator (always '.')
  - os.altsep is the alternate pathname separator (None or '/')
  - os.pathsep is the component separator used in $PATH etc
  - os.linesep is the line separator in text files ('\r' or '\n' or '\r\n')
  - os.defpath is the default search path for executables
  - os.devnull is the file path of the null device ('/dev/null', etc.)

Programs that import and use 'os' stand a better chance of being
portable between different platforms.  Of course, they must then
only use functions that are defined by all platforms (e.g., unlink
and opendir), and leave all pathname manipulation to os.path
(e.g., split and join).

cpu_count
	 <built-in function cpu_count>
_putenv
	 <built-in function putenv>
O_TEMPORARY
	 64
terminal_size
	 <class 'os.terminal_size'>
P_DETACH
	 4
__name__
	 os
dup
	 <built-in function dup>
getlogin
	 <built-in function getlogin>
fsdecode
	 <function _fscodec.<locals>.fsdecode at 0x01749B28>
get_handle_inheritable
	 <built-in function get_handle_inheritable>
close
	 <built-in function close>
altsep
	 /
utime
	 <built-in function utime>
open
	 <built-in function open>
execlpe
	 <function execlpe at 0x01732B70>
fdopen
	 <function fdopen at 0x01749C00>
supports_bytes_environ
	 False
unlink
	 <built-in function unlink>
execle
	 <function execle at 0x01732AE0>
system
	 <built-in function system>
renames
	 <function renames at 0x01732A08>
spawnle
	 <function spawnle at 0x01749B70>
O_RDONLY
	 0
stat_result
	 <class 'os.stat_result'>
fsencode
	 <function _fscodec.<locals>.fsencode at 0x01749AE0>
removedirs
	 <function removedirs at 0x017329C0>
uname_result
	 <class 'nt.uname_result'>
_unsetenv
	 <function <lambda> at 0x01732CD8>
abort
	 <built-in function abort>
umask
	 <built-in function umask>
_exit
	 <built-in function _exit>
error
	 <class 'OSError'>
getpid
	 <built-in function getpid>
O_SEQUENTIAL
	 32
walk
	 <function walk at 0x01732A50>
readlink
	 <built-in function readlink>
supports_dir_fd
	 set()
__file__
	 C:\Users\smszw\Anaconda3\lib\os.py
F_OK
	 0
P_OVERLAY
	 2
times_result
	 <class 'nt.times_result'>
set_handle_inheritable
	 <built-in function set_handle_inheritable>
kill
	 <built-in function kill>
getcwd
	 <built-in function getcwd>
fsync
	 <built-in function fsync>
pipe
	 <built-in function pipe>
TMP_MAX
	 32767
statvfs_result
	 <class 'os.statvfs_result'>
O_EXCL
	 1024
getcwdb
	 <built-in function getcwdb>
extsep
	 .
st
	 <module 'stat' from 'C:\\Users\\smszw\\Anaconda3\\lib\\stat.py'>
symlink
	 <built-in function symlink>
supports_follow_symlinks
	 {<built-in function stat>}
sep
	 \
rmdir
	 <built-in function rmdir>
closerange
	 <built-in function closerange>
stat
	 <built-in function stat>
errno
	 <module 'errno' (built-in)>
O_NOINHERIT
	 128
__loader__
	 <_frozen_importlib.SourceFileLoader object at 0x0171E650>
P_NOWAIT
	 1
environ
	 environ({'JAVA_HOME': 'C:\\Program Files\\Java\\jdk1.8.0_60', 'FP_NO_HOST_CHECK': 'NO', 'USERDOMAIN': 'user-PC', 'PATH': 'C:\\ProgramData\\Oracle\\Java\\javapath;C:\\Windows\\system32;C:\\Windows;C:\\Windows\\System32\\Wbem;C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\;C:\\Program Files\\Git\\cmd;C:\\Program Files\\Microsoft SQL Server\\110\\Tools\\Binn\\;C:\\Program Files\\Microsoft SQL Server\\110\\DTS\\Binn\\;C:\\Program Files\\Microsoft SQL Server\\110\\Tools\\Binn\\ManagementStudio\\;C:\\Program Files\\nodejs\\;C:\\Program Files\\Brackets\\command;%USERPROFILE%\\.dnx\\bin;C:\\Program Files\\Microsoft DNX\\Dnvm\\;C:\\Users\\smszw\\Anaconda2\\Library\\bin;C:\\Program Files\\IronPython 2.7;C:\\Users\\smszw\\Anaconda3;C:\\Users\\smszw\\Anaconda3\\Scripts; C:\\apache-ant-1.9.6\\bin;C:\\Users\\smszw\\AppData\\Roaming\\npm', 'CLICOLOR': '1', 'HOMEPATH': '\\Users\\smszw', 'PAGER': 'cat', 'COMPUTERNAME': 'USER-PC', 'LOGONSERVER': '\\\\USER-PC', 'IPY_INTERRUPT_EVENT': '956', 'SESSIONNAME': 'Console', 'PROMPT': '$P$G', 'PROCESSOR_IDENTIFIER': 'x86 Family 6 Model 15 Stepping 6, GenuineIntel', 'COMSPEC': 'C:\\Windows\\system32\\cmd.exe', 'PROCESSOR_REVISION': '0f06', 'PATHEXT': '.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC', 'VS140COMNTOOLS': 'C:\\Program Files\\Microsoft Visual Studio 14.0\\Common7\\Tools\\', 'NUMBER_OF_PROCESSORS': '2', 'TEMP': 'C:\\Users\\smszw\\AppData\\Local\\Temp', 'VBOX_MSI_INSTALL_PATH': 'C:\\Program Files\\Oracle\\VirtualBox\\', 'USERPROFILE': 'C:\\Users\\smszw', 'WINDOWS_TRACING_FLAGS': '3', 'JPY_PARENT_PID': '904', 'USERNAME': 'smszw', 'OS': 'Windows_NT', 'SYSTEMDRIVE': 'C:', 'SYSTEMROOT': 'C:\\Windows', 'JPY_INTERRUPT_EVENT': '956', 'APPDATA': 'C:\\Users\\smszw\\AppData\\Roaming', 'PUBLIC': 'C:\\Users\\Public', 'PROGRAMFILES': 'C:\\Program Files', 'PROCESSOR_ARCHITECTURE': 'x86', 'TERM': 'xterm-color', 'WINDOWS_TRACING_LOGFILE': 'C:\\BVTBin\\Tests\\installpackage\\csilogfile.log', 'COMMONPROGRAMFILES': 'C:\\Program Files\\Common Files', 'TMP': 'C:\\Users\\smszw\\AppData\\Local\\Temp', 'GIT_PAGER': 'cat', 'WINDIR': 'C:\\Windows', 'LOCALAPPDATA': 'C:\\Users\\smszw\\AppData\\Local', 'PROCESSOR_LEVEL': '6', 'ASL.LOG': 'Destination=file', 'PROGRAMDATA': 'C:\\ProgramData', 'ALLUSERSPROFILE': 'C:\\ProgramData', 'HOMEDRIVE': 'C:', 'PSMODULEPATH': 'C:\\Windows\\system32\\WindowsPowerShell\\v1.0\\Modules\\;C:\\Program Files\\Microsoft SQL Server\\110\\Tools\\PowerShell\\Modules\\'})
_exists
	 <function _exists at 0x01717A98>
__cached__
	 C:\Users\smszw\Anaconda3\lib\__pycache__\os.cpython-34.pyc
SEEK_SET
	 0
times
	 <built-in function times>
replace
	 <built-in function replace>
getenv
	 <function getenv at 0x017499C0>
strerror
	 <built-in function strerror>
execvp
	 <function execvp at 0x01732BB8>
_Environ
	 <class 'os._Environ'>
lseek
	 <built-in function lseek>
O_TRUNC
	 512
O_APPEND
	 8
mkdir
	 <built-in function mkdir>
path
	 <module 'ntpath' from 'C:\\Users\\smszw\\Anaconda3\\lib\\ntpath.py'>
putenv
	 <built-in function putenv>
__spec__
	 ModuleSpec(name='os', loader=<_frozen_importlib.SourceFileLoader object at 0x0171E650>, origin='C:\\Users\\smszw\\Anaconda3\\lib\\os.py')
startfile
	 <built-in function startfile>
_get_exports_list
	 <function _get_exports_list at 0x01717AE0>
waitpid
	 <built-in function waitpid>

Reloading Modules¶

reload currently only works on modules written in Python.

In [5]:

import os

from imp import reload  # Python3

reload(os)

Out[5]:

<module 'os' from 'C:\\Users\\smszw\\Anaconda3\\lib\\os.py'>

reload expects a module object.
Thus, the module you want to reload must first be successfully imported..
reload change the module object in-place.
Thus, every reference to it will be updated by a reload
from won't be affected by reload
- reload should be used with import not from
reload applys to a single module, not recursive
reload returns the module object
reload also accepts a dotted path name

Why care about reloading modules?¶

Dynamic customization.
Useful in larger systems, especially when the cost of restarting the entire application.
e.g. game servers, systems that must connect to servers over a network