I am a little confused by the multitude of ways in which you can import modules in Python.
import X
import X as Y
from A import B
I have been reading up about scoping and namespaces, but I would like some practical advice on what is the best strategy, under which circumstances and why. Should imports happen at a module level or a method/function level? In the __init__.py or in the module code itself?
My question is not really answered by "Python packages - import by class, not file" although it is obviously related.
In production code in our company, we try to follow the following rules.
We place imports at the beginning of the file, right after the main file's docstring, e.g.:
"""
Registry related functionality.
"""
import wx
# ...
Now, if we import a class that is one of few in the imported module, we import the name directly, so that in the code we only have to use the last part, e.g.:
from RegistryController import RegistryController
from ui.windows.lists import ListCtrl, DynamicListCtrl
There are modules, however, that contain dozens of classes, e.g. list of all possible exceptions. Then we import the module itself and reference to it in the code:
from main.core import Exceptions
# ...
raise Exceptions.FileNotFound()
We use the import X as Y as rarely as possible, because it makes searching for usage of a particular module or class difficult. Sometimes, however, you have to use it if you wish to import two classes that have the same name, but exist in different modules, e.g.:
from Queue import Queue
from main.core.MessageQueue import Queue as MessageQueue
As a general rule, we don't do imports inside methods -- they simply make code slower and less readable. Some may find this a good way to easily resolve cyclic imports problem, but a better solution is code reorganization.
Let me just paste a part of conversation on django-dev mailing list started by Guido van Rossum:
[...]
For example, it's part of the Google Python style guides[1] that all
imports must import a module, not a class or function from that
module. There are way more classes and functions than there are
modules, so recalling where a particular thing comes from is much
easier if it is prefixed with a module name. Often multiple modules
happen to define things with the same name -- so a reader of the code
doesn't have to go back to the top of the file to see from which
module a given name is imported.
Source: http://groups.google.com/group/django-developers/browse_thread/thread/78975372cdfb7d1a
1: http://code.google.com/p/soc/wiki/PythonStyleGuide#Module_and_package_imports
I would normally use import X on module level. If you only need a single object from a module, use from X import Y.
Only use import X as Y in case you're otherwise confronted with a name clash.
I only use imports on function level to import stuff I need when the module is used as the main module, like:
def main():
import sys
if len(sys.argv) > 1:
pass
HTH
Someone above said that
from X import A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P
is equivalent to
import X
import X allows direct modifications to A-P, while from X import ... creates copies of A-P. For from X import A..P you do not get updates to variables if they are modified. If you modify them, you only modify your copy, but X does know about your modifications.
If A-P are functions, you won't know the difference.
Others have covered most of the ground here but I just wanted to add one case where I will use import X as Y (temporarily), when I'm trying out a new version of a class or module.
So if we were migrating to a new implementation of a module, but didn't want to cut the code base over all at one time, we might write a xyz_new module and do this in the source files that we had migrated:
import xyz_new as xyz
Then, once we cut over the entire code base, we'd just replace the xyz module with xyz_new and change all of the imports back to
import xyz
DON'T do this:
from X import *
unless you are absolutely sure that you will use each and every thing in that module. And even then, you should probably reconsider using a different approach.
Other than that, it's just a matter of style.
from X import Y
is good and saves you lots of typing. I tend to use that when I'm using something in it fairly frequently But if you're importing a lot from that module, you could end up with an import statement that looks like this:
from X import A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P
You get the idea. That's when imports like
import X
become useful. Either that or if I'm not really using anything in X very frequently.
I generally try to use the regular import modulename, unless the module name is long, or used often..
For example, I would do..
from BeautifulSoup import BeautifulStoneSoup as BSS
..so I can do soup = BSS(html) instead of BeautifulSoup.BeautifulStoneSoup(html)
Or..
from xmpp import XmppClientBase
..instead of importing the entire of xmpp when I only use the XmppClientBase
Using import x as y is handy if you want to import either very long method names , or to prevent clobbering an existing import/variable/class/method (something you should try to avoid completely, but it's not always possible)
Say I want to run a main() function from another script, but I already have a main() function..
from my_other_module import main as other_module_main
..wouldn't replace my main function with my_other_module's main
Oh, one thing - don't do from x import * - it makes your code very hard to understand, as you cannot easily see where a method came from (from x import *; from y import *; my_func() - where is my_func defined?)
In all cases, you could just do import modulename and then do modulename.subthing1.subthing2.method("test")...
The from x import y as z stuff is purely for convenience - use it whenever it'll make your code easier to read or write!
When you have a well-written library, which is sometimes case in python, you ought just import it and use it as it. Well-written library tends to take life and language of its own, resulting in pleasant-to-read -code, where you rarely reference the library. When a library is well-written, you ought not need renaming or anything else too often.
import gat
node = gat.Node()
child = node.children()
Sometimes it's not possible to write it this way, or then you want to lift down things from library you imported.
from gat import Node, SubNode
node = Node()
child = SubNode(node)
Sometimes you do this for lot of things, if your import string overflows 80 columns, It's good idea to do this:
from gat import (
Node, SubNode, TopNode, SuperNode, CoolNode,
PowerNode, UpNode
)
The best strategy is to keep all of these imports on the top of the file. Preferrably ordered alphabetically, import -statements first, then from import -statements.
Now I tell you why this is the best convention.
Python could perfectly have had an automatic import, which'd look from the main imports for the value when it can't be found from global namespace. But this is not a good idea. I explain shortly why. Aside it being more complicated to implement than simple import, programmers wouldn't be so much thinking about the depedencies and finding out from where you imported things ought be done some other way than just looking into imports.
Need to find out depedencies is one reason why people hate "from ... import *". Some bad examples where you need to do this exist though, for example opengl -wrappings.
So the import definitions are actually valuable as defining the depedencies of the program. It is the way how you should exploit them. From them you can quickly just check where some weird function is imported from.
The import X as Y is useful if you have different implementations of the same module/class.
With some nested try..import..except ImportError..imports you can hide the implementation from your code. See lxml etree import example:
try:
from lxml import etree
print("running with lxml.etree")
except ImportError:
try:
# Python 2.5
import xml.etree.cElementTree as etree
print("running with cElementTree on Python 2.5+")
except ImportError:
try:
# Python 2.5
import xml.etree.ElementTree as etree
print("running with ElementTree on Python 2.5+")
except ImportError:
try:
# normal cElementTree install
import cElementTree as etree
print("running with cElementTree")
except ImportError:
try:
# normal ElementTree install
import elementtree.ElementTree as etree
print("running with ElementTree")
except ImportError:
print("Failed to import ElementTree from any known place")
I'm with Jason in the fact of not using
from X import *
But in my case (i'm not an expert programmer, so my code does not meet the coding style too well) I usually do in my programs a file with all the constants like program version, authors, error messages and all that stuff, so the file are just definitions, then I make the import
from const import *
That saves me a lot of time. But it's the only file that has that import, and it's because all inside that file are just variable declarations.
Doing that kind of import in a file with classes and definitions might be useful, but when you have to read that code you spend lots of time locating functions and classes.
Related
I'm building a dependency graph in python3 using the ast module. How do I know what file(s) will be imported if that import statement were to be executed?
Not a complete answer, but here are some bits you should be aware of:
Imports might happen in conditional or try-catch blocks. So depending on a setting of an environment variable, module A might or might not import module B.
There's a wide variety of import syntax: import A, from A import B, from A import *, from . import A, from .. import A, from ..A import B as well as their versions with A replaced with sub-modules.
Imports can happen in any executable context - the top-level of the file, in a function, in a class definition etc.
eval can evaluate code with imports. Up to you if you consider such code to be a dependency.
The standard library modulefinder module might help.
As suggested in a comment: the other answers are valid, but one of the fundamental problems is that your examples only work for 'simple' scripts or files: A lot of more complex code will use things like dynamic imports: consider the following:
path, task_name = "module.function".rsplit(".", 1);
module = importlib.import_module(path);
real_func = getattr(module, task_name);
real_func();
The actual original string could be obfuscated, or pulled from a DB, or a file or...
There are alternatives to importlib, but this is on top of the exec type stuff you might see in #horia's good answer.
I'm currently re-factoring a project (formerly big one file) into several seperate python files, each of which runs a specific part of my application.
Eg, GUIthread.py runs the GUI, Computethread.py does some maths, etc etc.
Each thread includes the use of functions from imported modules like math, time, numpy, etc etc.
I already have a file globalClasses.py containing class definitions for my datatypes etc, which each .py file imports at the start, as per recomendation here: http://effbot.org/pyfaq/how-do-i-share-global-variables-across-modules.htm . This is working well.
What I would like to do is have all my 3rdparty module imports in the globals file as well, so that I can write, for example, import math once but have all of my project files able to use math functions.
Questions:
1. Is this possible?
2. Is it a good idea/is it good Python practice?
My current solution is just to put
import math
import time
import numpy
...
(plus imports for all the other modules I'm using as well)
at the top of every file in my project... But that doesn't seem very tidy, and it's easy to forget to move a dependency's import statement when moving code-chunks from file to file...
Yeah I guess there is a more elegant way of doing this which will save redundant line of code. Suppose you want to import some modules math, time, numpy(say), then you can create a file importing_modules(say) and import the various modules as from module_name import *, So the importing_modules.py may look something like this:
importing_modules.py
from math import *
from numpy import *
from time import *
main.py
from importing_modules import *
#Now you can call the methods of that module directly
print sqrt(25) #Now we can call sqrt() directly in place of math.sqrt() or importing_modules.math.sqrt().
The other answer shows how what you want is (sort of) possible, but didn't address your second question about good practice.
Using import * is almost invariably considered bad practice. See "Why is import * bad?" and "Importing * from a package" from the docs.
Remember from PEP 20 that explicit is better than implicit. With explicit, specific imports (e.g. from math import sqrt) in every module, there is never confusion about from where a name came, your module's namespace includes only what it needs, and bugs are prevented.
The downside of having to write a couple import statements per module does not outweigh the potential problems introduced by trying to get around writing them.
I am new to Python as I want to expand skills that I learned using R.
In R I tend to load a bunch of libraries, sometimes resulting in function name conflicts.
What is best practice in Python. I have seen some specific variations that I do not see a difference between
import pandas, from pandas import *, and from pandas import DataFrame
What are the differences between the first two and should I just import what I need.
Also, what would be the worst consequences for someone making small programs to process data and compute simple statistics.
UPDATE
I found this excellent guide. It explains everything.
Disadvantage of each form
When reading other people's code (and those people use very
different importing styles), I noticed the following problems with
each of the styles:
import modulewithaverylongname will clutter the code further down
with the long module name (e.g. concurrent.futures or django.contrib.auth.backends) and decrease readability in those places.
from module import * gives me no chance to see syntactically that,
for instance, classA and classB come from the same module and
have a lot to do with each other.
It makes reading the code hard.
(That names from such an import
may shadow names from an earlier import is the least part of that problem.)
from module import classA, classB, functionC, constantD, functionE
overloads my short-term memory with too many names
that I mentally need to assign to module in order to
coherently understand the code.
import modulewithaverylongname as mwvln is sometimes insufficiently
mnemonic to me.
A suitable compromise
Based on the above observations, I have developed the following
style in my own code:
import module is the preferred style if the module name is short
as for example most of the packages in the standard library.
It is also the preferred style if I need to use names from the module in
only two or three places in my own module;
clarity trumps brevity then ("Readability counts").
import longername as ln is the preferred style in almost every
other case.
For instance, I might import django.contrib.auth.backends as djcab.
By definition of criterion 1 above, the abbreviation will be used
frequently and is therefore sufficiently easy to memorize.
Only these two styles are fully pythonic as per the
"Explicit is better than implicit." rule.
from module import xx still occurs sometimes in my code.
I use it in cases where even the as format appears exaggerated,
the most famous example being from datetime import datetime
(but if I need more elements, I will import datetime as dt).
import pandas imports the pandas module under the pandas namespace, so you would need to call objects within pandas using pandas.foo.
from pandas import * imports all objects from the pandas module into your current namespace, so you would call objects within pandas using only foo. Keep in mind this could have unexepcted consequences if there are any naming conflicts between your current namespace and the pandas namespace.
from pandas import DataFrame is the same as above, but only imports DataFrame (instead of everything) into your current namespace.
In my opinion the first is generally best practice, as it keeps the different modules nicely compartmentalized in your code.
Here are some recommendations from PEP8 Style Guide.
Imports should usually be on separate lines, e.g.:
Yes: import os
import sys
No: import sys, os
but it is okay to
from subprocess import Popen, PIPE
Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants.
Imports should be grouped in the following order:
standard library imports
related third party imports
local application/library specific imports
You should put a blank line between each group of imports.
Absolute imports are recommended
They are more readable and make debugging easier by giving better error messages in case you mess up import system.
import mypkg.sibling
from mypkg import sibling
from mypkg.sibling import example
or explicit relative imports
from . import sibling
from .sibling import example
Implicit relative imports should never be used and is removed in Python 3.
No: from ..grand_parent_package import uncle_package
Wildcard imports ( from <module> import * ) should be avoided, as they make it unclear which names are present in the namespace, confusing both readers and many automated tools.
Some recommendations about lazy imports from python speed performance tips.
Import Statement Overhead
import statements can be executed just about anywhere. It's often useful to place them inside functions to restrict their visibility and/or reduce initial startup time. Although Python's interpreter is optimized to not import the same module multiple times, repeatedly executing an import statement can seriously affect performance in some circumstances.
the given below is a scenario explained at the page,
>>> def doit1():
... import string
... string.lower('Python')
...
>>> import string
>>> def doit2():
... string.lower('Python')
...
>>> import timeit
>>> t = timeit.Timer(setup='from __main__ import doit1', stmt='doit1()')
>>> t.timeit()
11.479144930839539
>>> t = timeit.Timer(setup='from __main__ import doit2', stmt='doit2()')
>>> t.timeit()
4.6661689281463623
In general it is better to do explicit imports.
As in:
import pandas
frame = pandas.DataFrame()
Or:
from pandas import DataFrame
frame = DataFrame()
Another option in Python, when you have conflicting names, is import x as y:
from pandas import DataFrame as PDataFrame
from bears import DataFrame as BDataFrame
frame1 = PDataFrame()
frame2 = BDataFrame()
from A import B
essentially equals following three statements
import A
B = A.B
del A
That's it, that is it all.
They are all suitable in different contexts (which is why they are all available). There's no deep guiding principle, other than generic motherhood statements around clarity, maintainability and simplicity. Some examples from my own code:
import sys, os, re, itertools avoids name collisions and provides a very succinct way to import a bunch of standard modules.
from math import * lets me write sin(x) instead of math.sin(x) in math-heavy code. This gets a bit dicey when I also import numpy, which doubles up on some of these, but it doesn't overly concern me, since they are generally the same functions anyway. Also, I tend to follow the numpy documentation — import numpy as np — which sidesteps the issue entirely.
I favour from PIL import Image, ImageDraw just because that's the way the PIL documentation presents its examples.
I am interacting with a python 2.x API written in a non-OO way, it uses module-global scope for some internal state driven stuff. It's needed in a context where it's no longer singleton, and modifying the original code (not ours) is not an option.
Short of using subprocess runs of separate interpreters, is there any way I could box off the modules and interact with multiple instances of the module (thus treating it as an object)?
I need to use the module to drive 2 different setups - which it doesn't internally seem to work with.
Disclaimer: Please don't do this. Please do this only if in a very odd situation - and try to alter the situation in other ways before doing this. I did this to cope with odd code that could not be changed at the time of asking - not to provide a way to proliferate more odd code.
Just remove the module from sys.modules:
>>> import sys
>>> import mod as m1
>>> m1.x = 1
>>> del sys.modules['mod']
>>> import mod as m2
>>> m2.x = 2
>>> m1.x
1
You can try by fooling sys.modules
import badmodule as badmod1
import sys
del sys.modules['badmodule']
import badmodule as badmod2
If this works or not of course depends on what the bad module is doing...
I haven't used it personally but it seems that Exocet library may help.
Easiest way is to make two copies of the module and import them separately. For example, take your module thingabobber and make two copies named thingabobber1 and thingabobber2. Then just:
import thingabobber1, thingabobber2
If this isn't feasible, delete the module from sys.modules after initially importing it so you get a second copy on the second import.
import sys
import thingabobber as thingabobber1
del sys.modules["thingabobber"]
import thingabobber as thingabobber2
This can be achieved by importing the module via different paths.
That is - if in your sys.path you have two different dotted routes to the module, the module cache will create two different instances of the module with different symbol trees, globals and so on.
This can be used to have multiple versions of a library also.
Beware that it will lead to exceptions not being caught (as you are trying to catch the wrong symbol).
I was trying this with binary (so) submodules, but that procedure failed.
I'm working on pypreprocessor which is a preprocessor that takes c-style directives and I've been able to make it work like a traditional preprocessor (it's self-consuming and executes postprocessed code on-the-fly) except that it breaks library imports.
The problem is: The preprocessor runs through the file, processes it, outputs to a temporary file, and exec() the temporary file. Libraries that are imported need to be handled a little different, because they aren't executed, but rather they are loaded and made accessible to the caller module.
What I need to be able to do is: Interrupt the import (since the preprocessor is being run in the middle of the import), load the postprocessed code as a tempModule, and replace the original import with the tempModule to trick the calling script with the import into believing that the tempModule is the original module.
I have searched everywhere and so far and have no solution.
This Stack Overflow question is the closest I've seen so far to providing an answer:
Override namespace in Python
Here's what I have.
# Remove the bytecode file created by the first import
os.remove(moduleName + '.pyc')
# Remove the first import
del sys.modules[moduleName]
# Import the postprocessed module
tmpModule = __import__(tmpModuleName)
# Set first module's reference to point to the preprocessed module
sys.modules[moduleName] = tmpModule
moduleName is the name of the original module, and tmpModuleName is the name of the postprocessed code file.
The strange part is this solution still runs completely normal as if the first module completed loaded normally; unless you remove the last line, then you get a module not found error.
Hopefully someone on Stack Overflow know a lot more about imports than I do, because this one has me stumped.
Note: I will only award a solution, or, if this is not possible in Python; the best, most detailed explanation of why this is not impossible.
Update: For anybody who is interested, here is the working code.
if imp.lock_held() is True:
del sys.modules[moduleName]
sys.modules[tmpModuleName] = __import__(tmpModuleName)
sys.modules[moduleName] = __import__(tmpModuleName)
The 'imp.lock_held' part detects whether the module is being loaded as a library. The following lines do the rest.
Does this answer your question? The second import does the trick.
Mod_1.py
def test_function():
print "Test Function -- Mod 1"
Mod_2.py
def test_function():
print "Test Function -- Mod 2"
Test.py
#!/usr/bin/python
import sys
import Mod_1
Mod_1.test_function()
del sys.modules['Mod_1']
sys.modules['Mod_1'] = __import__('Mod_2')
import Mod_1
Mod_1.test_function()
To define a different import behavior or to totally subvert the import process you will need to write import hooks. See PEP 302.
For example,
import sys
class MyImporter(object):
def find_module(self, module_name, package_path):
# Return a loader
return self
def load_module(self, module_name):
# Return a module
return self
sys.meta_path.append(MyImporter())
import now_you_can_import_any_name
print now_you_can_import_any_name
It outputs:
<__main__.MyImporter object at 0x009F85F0>
So basically it returns a new module (which can be any object), in this case itself. You may use it to alter the import behavior by returning processe_xxx on import of xxx.
IMO: Python doesn't need a preprocessor. Whatever you are accomplishing can be accomplished in Python itself due to it very dynamic nature, for example, taking the case of the debug example, what is wrong with having at top of file
debug = 1
and later
if debug:
print "wow"
?
In Python 2 there is the imputil module that seems to provide the functionality you are looking for, but has been removed in python 3. It's not very well documented but contains an example section that shows how you can replace the standard import functions.
For Python 3 there is the importlib module (introduced in Python 3.1) that contains functions and classes to modify the import functionality in all kinds of ways. It should be suitable to hook your preprocessor into the import system.