To understand what yield
does, you must understand what generators are. And before you can understand generators, you must understand iterables.
Iterables
When you create a list, you can read its items one by one. Reading its items one by one is called iteration:
>>> mylist = [1, 2, 3]
>>> for i in mylist:
... print(i)
1
2
3
mylist
is an iterable. When you use a list comprehension, you create a list, and so an iterable:
>>> mylist = [x*x for x in range(3)]
>>> for i in mylist:
... print(i)
0
1
4
Everything you can use "for... in...
" on is an iterable; lists
, strings
, files...
These iterables are handy because you can read them as much as you wish, but you store all the values in memory and this is not always what you want when you have a lot of values.
Generators
Generators are iterators, a kind of iterable you can only iterate over once. Generators do not store all the values in memory, they generate the values on the fly:
>>> mygenerator = (x*x for x in range(3))
>>> for i in mygenerator:
... print(i)
0
1
4
It is just the same except you used ()
instead of []
. BUT, you cannot perform for i in mygenerator
a second time since generators can only be used once: they calculate 0, then forget about it and calculate 1, and end calculating 4, one by one.
Yield
yield
is a keyword that is used like return
, except the function will return a generator.
>>> def create_generator():
... mylist = range(3)
... for i in mylist:
... yield i*i
...
>>> mygenerator = create_generator() # create a generator
>>> print(mygenerator) # mygenerator is an object!
<generator object create_generator at 0xb7555c34>
>>> for i in mygenerator:
... print(i)
0
1
4
Here it's a useless example, but it's handy when you know your function will return a huge set of values that you will only need to read once.
To master yield
, you must understand that when you call the function, the code you have written in the function body does not run. The function only returns the generator object, this is a bit tricky.
Then, your code will continue from where it left off each time for
uses the generator.
Now the hard part:
The first time the for
calls the generator object created from your function, it will run the code in your function from the beginning until it hits yield
, then it'll return the first value of the loop. Then, each subsequent call will run another iteration of the loop you have written in the function and return the next value. This will continue until the generator is considered empty, which happens when the function runs without hitting yield
. That can be because the loop has come to an end, or because you no longer satisfy an "if/else"
.
Your code explained
Generator:
# Here you create the method of the node object that will return the generator
def _get_child_candidates(self, distance, min_dist, max_dist):
# Here is the code that will be called each time you use the generator object:
# If there is still a child of the node object on its left
# AND if the distance is ok, return the next child
if self._leftchild and distance - max_dist < self._median:
yield self._leftchild
# If there is still a child of the node object on its right
# AND if the distance is ok, return the next child
if self._rightchild and distance + max_dist >= self._median:
yield self._rightchild
# If the function arrives here, the generator will be considered empty
# there is no more than two values: the left and the right children
Caller:
# Create an empty list and a list with the current object reference
result, candidates = list(), [self]
# Loop on candidates (they contain only one element at the beginning)
while candidates:
# Get the last candidate and remove it from the list
node = candidates.pop()
# Get the distance between obj and the candidate
distance = node._get_dist(obj)
# If distance is ok, then you can fill the result
if distance <= max_dist and distance >= min_dist:
result.extend(node._values)
# Add the children of the candidate in the candidate's list
# so the loop will keep running until it will have looked
# at all the children of the children of the children, etc. of the candidate
candidates.extend(node._get_child_candidates(distance, min_dist, max_dist))
return result
This code contains several smart parts:
The loop iterates on a list, but the list expands while the loop is being iterated. It's a concise way to go through all these nested data even if it's a bit dangerous since you can end up with an infinite loop. In this case, candidates.extend(node._get_child_candidates(distance, min_dist, max_dist))
exhaust all the values of the generator, but while
keeps creating new generator objects which will produce different values from the previous ones since it's not applied on the same node.
The extend()
method is a list object method that expects an iterable and adds its values to the list.
Usually we pass a list to it:
>>> a = [1, 2]
>>> b = [3, 4]
>>> a.extend(b)
>>> print(a)
[1, 2, 3, 4]
But in your code, it gets a generator, which is good because:
- You don't need to read the values twice.
- You may have a lot of children and you don't want them all stored in memory.
And it works because Python does not care if the argument of a method is a list or not. Python expects iterables so it will work with strings, lists, tuples, and generators! This is called duck typing and is one of the reasons why Python is so cool. But this is another story, for another question...
You can stop here, or read a little bit to see an advanced use of a generator:
Controlling a generator exhaustion
>>> class Bank(): # Let's create a bank, building ATMs
... crisis = False
... def create_atm(self):
... while not self.crisis:
... yield "$100"
>>> hsbc = Bank() # When everything's ok the ATM gives you as much as you want
>>> corner_street_atm = hsbc.create_atm()
>>> print(corner_street_atm.next())
$100
>>> print(corner_street_atm.next())
$100
>>> print([corner_street_atm.next() for cash in range(5)])
['$100', '$100', '$100', '$100', '$100']
>>> hsbc.crisis = True # Crisis is coming, no more money!
>>> print(corner_street_atm.next())
<type 'exceptions.StopIteration'>
>>> wall_street_atm = hsbc.create_atm() # It's even true for new ATMs
>>> print(wall_street_atm.next())
<type 'exceptions.StopIteration'>
>>> hsbc.crisis = False # The trouble is, even post-crisis the ATM remains empty
>>> print(corner_street_atm.next())
<type 'exceptions.StopIteration'>
>>> brand_new_atm = hsbc.create_atm() # Build a new one to get back in business
>>> for cash in brand_new_atm:
... print cash
$100
$100
$100
$100
$100
$100
$100
$100
$100
...
Note: For Python 3, useprint(corner_street_atm.__next__())
or print(next(corner_street_atm))
It can be useful for various things like controlling access to a resource.
Itertools, your best friend
The itertools module contains special functions to manipulate iterables. Ever wish to duplicate a generator?
Chain two generators? Group values in a nested list with a one-liner? Map / Zip
without creating another list?
Then just import itertools
.
An example? Let's see the possible orders of arrival for a four-horse race:
>>> horses = [1, 2, 3, 4]
>>> races = itertools.permutations(horses)
>>> print(races)
<itertools.permutations object at 0xb754f1dc>
>>> print(list(itertools.permutations(horses)))
[(1, 2, 3, 4),
(1, 2, 4, 3),
(1, 3, 2, 4),
(1, 3, 4, 2),
(1, 4, 2, 3),
(1, 4, 3, 2),
(2, 1, 3, 4),
(2, 1, 4, 3),
(2, 3, 1, 4),
(2, 3, 4, 1),
(2, 4, 1, 3),
(2, 4, 3, 1),
(3, 1, 2, 4),
(3, 1, 4, 2),
(3, 2, 1, 4),
(3, 2, 4, 1),
(3, 4, 1, 2),
(3, 4, 2, 1),
(4, 1, 2, 3),
(4, 1, 3, 2),
(4, 2, 1, 3),
(4, 2, 3, 1),
(4, 3, 1, 2),
(4, 3, 2, 1)]
Understanding the inner mechanisms of iteration
Iteration is a process implying iterables (implementing the __iter__()
method) and iterators (implementing the __next__()
method).
Iterables are any objects you can get an iterator from. Iterators are objects that let you iterate on iterables.
There is more about it in this article about how for
loops work.
Best Answer
Script vs. Module
Here's an explanation. The short version is that there is a big difference between directly running a Python file, and importing that file from somewhere else. Just knowing what directory a file is in does not determine what package Python thinks it is in. That depends, additionally, on how you load the file into Python (by running or by importing).
There are two ways to load a Python file: as the top-level script, or as a module. A file is loaded as the top-level script if you execute it directly, for instance by typing
python myfile.py
on the command line. It is loaded as a module when animport
statement is encountered inside some other file. There can only be one top-level script at a time; the top-level script is the Python file you ran to start things off.Naming
When a file is loaded, it is given a name (which is stored in its
__name__
attribute). If it was loaded as the top-level script, its name is__main__
. If it was loaded as a module, its name is the filename, preceded by the names of any packages/subpackages of which it is a part, separated by dots.So for instance in your example:
if you imported
moduleX
(note: imported, not directly executed), its name would bepackage.subpackage1.moduleX
. If you importedmoduleA
, its name would bepackage.moduleA
. However, if you directly runmoduleX
from the command line, its name will instead be__main__
, and if you directly runmoduleA
from the command line, its name will be__main__
. When a module is run as the top-level script, it loses its normal name and its name is instead__main__
.Accessing a module NOT through its containing package
There is an additional wrinkle: the module's name depends on whether it was imported "directly" from the directory it is in or imported via a package. This only makes a difference if you run Python in a directory, and try to import a file in that same directory (or a subdirectory of it). For instance, if you start the Python interpreter in the directory
package/subpackage1
and then doimport moduleX
, the name ofmoduleX
will just bemoduleX
, and notpackage.subpackage1.moduleX
. This is because Python adds the current directory to its search path when the interpreter is entered interactively; if it finds the to-be-imported module in the current directory, it will not know that that directory is part of a package, and the package information will not become part of the module's name.A special case is if you run the interpreter interactively (e.g., just type
python
and start entering Python code on the fly). In this case, the name of that interactive session is__main__
.Now here is the crucial thing for your error message: if a module's name has no dots, it is not considered to be part of a package. It doesn't matter where the file actually is on disk. All that matters is what its name is, and its name depends on how you loaded it.
Now look at the quote you included in your question:
Relative imports...
Relative imports use the module's name to determine where it is in a package. When you use a relative import like
from .. import foo
, the dots indicate to step up some number of levels in the package hierarchy. For instance, if your current module's name ispackage.subpackage1.moduleX
, then..moduleA
would meanpackage.moduleA
. For afrom .. import
to work, the module's name must have at least as many dots as there are in theimport
statement.... are only relative in a package
However, if your module's name is
__main__
, it is not considered to be in a package. Its name has no dots, and therefore you cannot usefrom .. import
statements inside it. If you try to do so, you will get the "relative-import in non-package" error.Scripts can't import relative
What you probably did is you tried to run
moduleX
or the like from the command line. When you did this, its name was set to__main__
, which means that relative imports within it will fail, because its name does not reveal that it is in a package. Note that this will also happen if you run Python from the same directory where a module is, and then try to import that module, because, as described above, Python will find the module in the current directory "too early" without realizing it is part of a package.Also remember that when you run the interactive interpreter, the "name" of that interactive session is always
__main__
. Thus you cannot do relative imports directly from an interactive session. Relative imports are only for use within module files.Two solutions:
If you really do want to run
moduleX
directly, but you still want it to be considered part of a package, you can dopython -m package.subpackage1.moduleX
. The-m
tells Python to load it as a module, not as the top-level script.Or perhaps you don't actually want to run
moduleX
, you just want to run some other script, saymyfile.py
, that uses functions insidemoduleX
. If that is the case, putmyfile.py
somewhere else – not inside thepackage
directory – and run it. If insidemyfile.py
you do things likefrom package.moduleA import spam
, it will work fine.Notes
For either of these solutions, the package directory (
package
in your example) must be accessible from the Python module search path (sys.path
). If it is not, you will not be able to use anything in the package reliably at all.Since Python 2.6, the module's "name" for package-resolution purposes is determined not just by its
__name__
attributes but also by the__package__
attribute. That's why I'm avoiding using the explicit symbol__name__
to refer to the module's "name". Since Python 2.6 a module's "name" is effectively__package__ + '.' + __name__
, or just__name__
if__package__
isNone
.)