I'm writing a couple Python modules to be used in my own application handling crypto-currencies. Many of the functions return something based upon a given string:
def doStuff(coin, value):
if coin == 'BTC':
return doSomethingWithBTC(value, 'some_string')
elif coin == 'LTC':
return someModule.doLTC(value, 1)
elif coin == 'DOGE':
return otherMod.DOGE(value, 52, True)
else:
return 'Some terrible error occurred.'
As you can see, the key is one of a predefined set of strings (crypto-currencies). At the moment there are three in the set, but I want to extend this number in the future. I've got a dozen more functions in this module, which at the moment all take the same three strings, but when I add one, all the functions need to be extended.
I've got unit tests for this module in which I now want to test whether all the functions are able to take all the (currently 3) items from the set as a key. I could do this by calling them, but some of them do stuff which I can't really test in unit tests. One of them makes (Bitcoin) payments for example.
I now thought of using the inspect module to get the source code of the functions and see if it contains a line containing key == 'X', where X is each item from the pre-defined set. Although I guess this will work, it doesn't sound very Pythonic to me.
Does anybody know how I can make these functions such that I can test whether they are able to handle all currencies in the pre-defined set without actually calling the functions? All tips are welcome!
Using if-cascades is not very pythonic either. If you have many methods for each key, just use classes and access them via a dictionary:
class BTC(object):
def do_stuff(self, value):
return doSomethingWithBTC(value, 'some_string')
class LTC(object):
...
CURRENCIE_INSTANCES = {
'BTC': BTC(),
'LTC': LTC(),
}
def do_stuff(key, value):
return CURRENCIE_INSTANCES[key].do_stuff(value)
That way, you don't have to change your stuff-methods, but only one dictionary.
How about this approach:
from functools import partial
def doStuff(key, value):
method_btc = partial(doSomethingWithBTC, kw2='some_string')
method_ltc = partial(someModule.doLTC, kw2=1)
method_doge = pattial(otherMod.DOGE, kw2=52, kw3=True)
dct = dict(BTC=method_btc, LTC=method_ltc, DOGE=method_doge)
if key in dct:
return dct.get(key)(value)
else:
return 'Some terrible error occurred.'
You can always expand dictionary dct with new key and method.
Related
I'm new to Python, and am sort of surprised I cannot do this.
dictionary = {
'a' : '123',
'b' : dictionary['a'] + '456'
}
I'm wondering what the Pythonic way to correctly do this in my script, because I feel like I'm not the only one that has tried to do this.
EDIT: Enough people were wondering what I'm doing with this, so here are more details for my use cases. Lets say I want to keep dictionary objects to hold file system paths. The paths are relative to other values in the dictionary. For example, this is what one of my dictionaries may look like.
dictionary = {
'user': 'sholsapp',
'home': '/home/' + dictionary['user']
}
It is important that at any point in time I may change dictionary['user'] and have all of the dictionaries values reflect the change. Again, this is an example of what I'm using it for, so I hope that it conveys my goal.
From my own research I think I will need to implement a class to do this.
No fear of creating new classes -
You can take advantage of Python's string formating capabilities
and simply do:
class MyDict(dict):
def __getitem__(self, item):
return dict.__getitem__(self, item) % self
dictionary = MyDict({
'user' : 'gnucom',
'home' : '/home/%(user)s',
'bin' : '%(home)s/bin'
})
print dictionary["home"]
print dictionary["bin"]
Nearest I came up without doing object:
dictionary = {
'user' : 'gnucom',
'home' : lambda:'/home/'+dictionary['user']
}
print dictionary['home']()
dictionary['user']='tony'
print dictionary['home']()
>>> dictionary = {
... 'a':'123'
... }
>>> dictionary['b'] = dictionary['a'] + '456'
>>> dictionary
{'a': '123', 'b': '123456'}
It works fine but when you're trying to use dictionary it hasn't been defined yet (because it has to evaluate that literal dictionary first).
But be careful because this assigns to the key of 'b' the value referenced by the key of 'a' at the time of assignment and is not going to do the lookup every time. If that is what you are looking for, it's possible but with more work.
What you're describing in your edit is how an INI config file works. Python does have a built in library called ConfigParser which should work for what you're describing.
This is an interesting problem. It seems like Greg has a good solution. But that's no fun ;)
jsbueno as a very elegant solution but that only applies to strings (as you requested).
The trick to a 'general' self referential dictionary is to use a surrogate object. It takes a few (understatement) lines of code to pull off, but the usage is about what you want:
S = SurrogateDict(AdditionSurrogateDictEntry)
d = S.resolve({'user': 'gnucom',
'home': '/home/' + S['user'],
'config': [S['home'] + '/.emacs', S['home'] + '/.bashrc']})
The code to make that happen is not nearly so short. It lives in three classes:
import abc
class SurrogateDictEntry(object):
__metaclass__ = abc.ABCMeta
def __init__(self, key):
"""record the key on the real dictionary that this will resolve to a
value for
"""
self.key = key
def resolve(self, d):
""" return the actual value"""
if hasattr(self, 'op'):
# any operation done on self will store it's name in self.op.
# if this is set, resolve it by calling the appropriate method
# now that we can get self.value out of d
self.value = d[self.key]
return getattr(self, self.op + 'resolve__')()
else:
return d[self.key]
#staticmethod
def make_op(opname):
"""A convience class. This will be the form of all op hooks for subclasses
The actual logic for the op is in __op__resolve__ (e.g. __add__resolve__)
"""
def op(self, other):
self.stored_value = other
self.op = opname
return self
op.__name__ = opname
return op
Next, comes the concrete class. simple enough.
class AdditionSurrogateDictEntry(SurrogateDictEntry):
__add__ = SurrogateDictEntry.make_op('__add__')
__radd__ = SurrogateDictEntry.make_op('__radd__')
def __add__resolve__(self):
return self.value + self.stored_value
def __radd__resolve__(self):
return self.stored_value + self.value
Here's the final class
class SurrogateDict(object):
def __init__(self, EntryClass):
self.EntryClass = EntryClass
def __getitem__(self, key):
"""record the key and return"""
return self.EntryClass(key)
#staticmethod
def resolve(d):
"""I eat generators resolve self references"""
stack = [d]
while stack:
cur = stack.pop()
# This just tries to set it to an appropriate iterable
it = xrange(len(cur)) if not hasattr(cur, 'keys') else cur.keys()
for key in it:
# sorry for being a duche. Just register your class with
# SurrogateDictEntry and you can pass whatever.
while isinstance(cur[key], SurrogateDictEntry):
cur[key] = cur[key].resolve(d)
# I'm just going to check for iter but you can add other
# checks here for items that we should loop over.
if hasattr(cur[key], '__iter__'):
stack.append(cur[key])
return d
In response to gnucoms's question about why I named the classes the way that I did.
The word surrogate is generally associated with standing in for something else so it seemed appropriate because that's what the SurrogateDict class does: an instance replaces the 'self' references in a dictionary literal. That being said, (other than just being straight up stupid sometimes) naming is probably one of the hardest things for me about coding. If you (or anyone else) can suggest a better name, I'm all ears.
I'll provide a brief explanation. Throughout S refers to an instance of SurrogateDict and d is the real dictionary.
A reference S[key] triggers S.__getitem__ and SurrogateDictEntry(key) to be placed in the d.
When S[key] = SurrogateDictEntry(key) is constructed, it stores key. This will be the key into d for the value that this entry of SurrogateDictEntry is acting as a surrogate for.
After S[key] is returned, it is either entered into the d, or has some operation(s) performed on it. If an operation is performed on it, it triggers the relative __op__ method which simple stores the value that the operation is performed on and the name of the operation and then returns itself. We can't actually resolve the operation because d hasn't been constructed yet.
After d is constructed, it is passed to S.resolve. This method loops through d finding any instances of SurrogateDictEntry and replacing them with the result of calling the resolve method on the instance.
The SurrogateDictEntry.resolve method receives the now constructed d as an argument and can use the value of key that it stored at construction time to get the value that it is acting as a surrogate for. If an operation was performed on it after creation, the op attribute will have been set with the name of the operation that was performed. If the class has a __op__ method, then it has a __op__resolve__ method with the actual logic that would normally be in the __op__ method. So now we have the logic (self.op__resolve) and all necessary values (self.value, self.stored_value) to finally get the real value of d[key]. So we return that which step 4 places in the dictionary.
finally the SurrogateDict.resolve method returns d with all references resolved.
That'a a rough sketch. If you have any more questions, feel free to ask.
If you, just like me wandering how to make #jsbueno snippet work with {} style substitutions, below is the example code (which is probably not much efficient though):
import string
class MyDict(dict):
def __init__(self, *args, **kw):
super(MyDict,self).__init__(*args, **kw)
self.itemlist = super(MyDict,self).keys()
self.fmt = string.Formatter()
def __getitem__(self, item):
return self.fmt.vformat(dict.__getitem__(self, item), {}, self)
xs = MyDict({
'user' : 'gnucom',
'home' : '/home/{user}',
'bin' : '{home}/bin'
})
>>> xs["home"]
'/home/gnucom'
>>> xs["bin"]
'/home/gnucom/bin'
I tried to make it work with the simple replacement of % self with .format(**self) but it turns out it wouldn't work for nested expressions (like 'bin' in above listing, which references 'home', which has it's own reference to 'user') because of the evaluation order (** expansion is done before actual format call and it's not delayed like in original % version).
Write a class, maybe something with properties:
class PathInfo(object):
def __init__(self, user):
self.user = user
#property
def home(self):
return '/home/' + self.user
p = PathInfo('thc')
print p.home # /home/thc
As sort of an extended version of #Tony's answer, you could build a dictionary subclass that calls its values if they are callables:
class CallingDict(dict):
"""Returns the result rather than the value of referenced callables.
>>> cd = CallingDict({1: "One", 2: "Two", 'fsh': "Fish",
... "rhyme": lambda d: ' '.join((d[1], d['fsh'],
... d[2], d['fsh']))})
>>> cd["rhyme"]
'One Fish Two Fish'
>>> cd[1] = 'Red'
>>> cd[2] = 'Blue'
>>> cd["rhyme"]
'Red Fish Blue Fish'
"""
def __getitem__(self, item):
it = super(CallingDict, self).__getitem__(item)
if callable(it):
return it(self)
else:
return it
Of course this would only be usable if you're not actually going to store callables as values. If you need to be able to do that, you could wrap the lambda declaration in a function that adds some attribute to the resulting lambda, and check for it in CallingDict.__getitem__, but at that point it's getting complex, and long-winded, enough that it might just be easier to use a class for your data in the first place.
This is very easy in a lazily evaluated language (haskell).
Since Python is strictly evaluated, we can do a little trick to turn things lazy:
Y = lambda f: (lambda x: x(x))(lambda y: f(lambda *args: y(y)(*args)))
d1 = lambda self: lambda: {
'a': lambda: 3,
'b': lambda: self()['a']()
}
# fix the d1, and evaluate it
d2 = Y(d1)()
# to get a
d2['a']() # 3
# to get b
d2['b']() # 3
Syntax wise this is not very nice. That's because of us needing to explicitly construct lazy expressions with lambda: ... and explicitly evaluate lazy expression with ...(). It's the opposite problem in lazy languages needing strictness annotations, here in Python we end up needing lazy annotations.
I think with some more meta-programmming and some more tricks, the above could be made more easy to use.
Note that this is basically how let-rec works in some functional languages.
The jsbueno answer in Python 3 :
class MyDict(dict):
def __getitem__(self, item):
return dict.__getitem__(self, item).format(self)
dictionary = MyDict({
'user' : 'gnucom',
'home' : '/home/{0[user]}',
'bin' : '{0[home]}/bin'
})
print(dictionary["home"])
print(dictionary["bin"])
Her ewe use the python 3 string formatting with curly braces {} and the .format() method.
Documentation : https://docs.python.org/3/library/string.html
I have a situation where I need to create a dictionary that keeps track of global order of the values. I haven't been able to find a good way for the class itself to have an incrementing counter that's also tracked by the value.
Here's what I've written in the meanwhile to get around this:
from collections import defaultdict
class NotMyDict(object):
""" defaultdict(list) that tracks order globally across the dict.
Will function as a normal defaultdict(list) unless you modify the
'ordered' attribute and set it to a non-false evaluating value. This
"""
ordered = False
_data = defaultdict(NotMyDictList)
_next_index = 0
class NotMyDictList(list):
def append(self, value):
def __repr__(self):
if self.ordered:
return repr(self._data)
else:
temp = defaultdict(list)
for key in self._data:
for value in self._data[key]:
temp[key].append(value[0])
return repr(temp)
def __getitem__(self, key):
if self.ordered:
return self._data[key]
else:
return [val[0] for val in self._data[key]]
def add_value_to_key(self, key, value):
self._data[key].append((value, self._next_index))
self._next_index += 1
So I can use this like a normal dictionary for pulling values. I could have instantiated a list if the key didn't exist, but defaultdict was simple and easy.
Here's an example of the use:
test = NotMyDict()
test.add_value_to_key('test', 'hi')
test.add_value_to_key('test', 'there')
test.add_value_to_key('test', 'buddy')
test['test']
Result:
['hi', 'there', 'buddy']
test.ordered = True
test['test']
Result:
[('hi', 0), ('there', 1), ('buddy', 2)]
Now - the example of use isn't super important, but the functionality that I can't seem to figure out, is instead of using the .add_value_to_key(), I want to be able to use a normal defaultdict(list) convention of:
dict[key].append()
and still have it track the index. Do I need to pass global object locations with id() and reference those objects at a memory level, or is there a way I just don't understand to have a "class global" that's accessible by it's members?
I had also tried to use nested classes, but the nested class didn't have access to the parent class's globals, so I'd have to:
Make a list-like class that references the parent class attribute somehow (Maybe with id() and direct memory location reference?)
modify/make it's append() function so that it also updates the parent class global counter, and tracks the value with this counter as a metadata field.
I really just can't seem to wrap my head around how to create this object/class in a way that let's me use the same functionality of a defaultdict(list) where I can index/append directly AND have it track the global index order of that new value.
dict[key].append(value)
Help would be appreciated - I sunk three hours into trying different solutions before I scrapped it and went with the "just use this method to append" for now.
I have a working solution to this question, it just doesn't feel very pythonic. I am working in Python 2.7 and, thus, cannot use Python 3 solutions.
I have a dictionary that is regularly being updated. Eventually a key, let's call it "foo", with a value will appear in the dictionary. I want to keep polling that object and getting that dictionary until the key "foo" appears at which point I want to get the value associated with that key and use it.
Here is some psuedo code that is functioning right now:
polled_dict = my_object.get_dict()
while('foo' not in polled_dict.keys()):
polled_dict = my_object.get_dict()
fooValue = polled_dict['foo']
Let me emphasize that what the code is doing right now works. It feels gross but it works. A potential saolution I came up with is:
fooValue = None
While fooValue is None:
polled_dict = my_object.get_dict()
fooValue = polled_dict.get('foo')
This also works but it only seems a tiny bit better. Instead of calling polled_dict.get('foo') twice once it shows up in the dict(the key is accessed during the while loop and again on exiting the while loop) we only call it once. But, honestly, it doesn't seem much better and the gains are minimal.
As I look over the other solutions I've implemented I see that they're just different logical permutations of the two above examples (a not in a different place or something) but nothing feels pythonic. I seems like there would be an easy, cleaner way of doing this. Any suggestions? If not, is either of the above better than the other?
EDIT A lot of answers are recommending I override or otherwise change the dictionaries that the code is polling from. I agree that this would normally be a great solution but, to quote from some of my comments below:
"The code in question needs to exist separately from the API that updates the dictionary. This code needs to be generic and access the dictionary of a large number of different types of objects. Adding a trigger would ultimately require completely reworking all of those objects (and would not be nearly as generic as this function needs to be) This is grossly simplified obviously but, ultimately, I need to check values in this dict until it shows up instead of triggering something in the object. I'm unconvinced that making such a wide reaching and potentially damaging change is a pythonic solution(though should the API be rewritten from the ground up this will definitely be the solution and for something that does not need to be separated/can access the API this is definitely the pythonic solution.)"
You could always do something like subclass dict.
This is completely untested, but something to the effect of:
class NoisyDict(dict):
def __init__(self, *args, **kwargs):
self.handlers = {}
#Python 3 style
super().__init__(*args, **kwargs)
def add_handler(self, key, callback):
self.handlers[key] = self.handlers.get(key, [])
self.handlers[key].append(callback)
def __getitem__(self, key):
for handler in self.handlers.get(key, []):
handler('get', key, super().__getitem__(key))
return super().__getitem__(key)
def __setitem__(self, key, value):
for handler in self.handlers.get(key, []):
handler('set', key, value)
return super().__setitem(value)
Then you could do
d = NoisyDict()
d.add_handler('spam', print)
d['bar'] = 3
d['spam'] = 'spam spam spam'
Fun with generators:
from itertools import repeat
gen_dict = (o.get_dict() for o in repeat(my_object))
foo_value = next(d['foo'] for d in gen_dict if 'foo' in d)
Is it not possible to do something like this? (obviously not thread safe) The only catch is that the method below does not catch dictionary initialization via construction. That is it wouldn't catch keys added when the dictionary is created; eg MyDict(watcher=MyWatcher(), a=1, b=2) - the a and b keys would not be caught as added. I'm not sure how to implement that.
class Watcher(object):
"""Watches entries added to a MyDict (dictionary). key_found() is called
when an item is added whose key matches one of elements in keys.
"""
def __init__(self, *keys):
self.keys = keys
def key_found(self, key, value):
print key, value
class MyDict(dict):
def __init__(self, *args, **kwargs):
self.watcher = kwargs.pop('watcher')
super(MyDict, self).__init__(*args, **kwargs)
def __setitem__(self, key, value):
super(MyDict, self).__setitem__(key, value)
if key in self.watcher.keys:
self.watcher.key_found(key, value)
watcher = Watcher('k1', 'k2', 'k3')
d = MyDict(watcher=watcher)
d['a'] = 1
d['b'] = 2
d['k1'] = 'k1 value'
If your object is modifying the dictionary in place then you should only need to get it once. Then you and your object have a pointer to the same dictionary object. If you need to stick with polling then this is probably the cleanest solution:
polled_dict = my_object.get_dict()
while 'foo' not in polled_dict:
pass # optionally sleep
fooValue = polled_dict['foo']
The best overall way of doing this would be to push some type of event through a pipe/socket/thread-lock in some way.
Maybe Try/Except would be considered more 'Pythonic'?
A sleep statement in the while loop will stop it consuming all your resources as well.
polled_dict = my_object.get_dict()
while True:
time.sleep(0.1)
try:
fooValue = polled_dict['foo']
return (foovalue) # ...or break
except KeyError:
polled_dict = my_object.get_dict()
I think a defaultdict is great for this kind of job.
from collections import defaultdict
mydeafultdict = defaultdict(lambda : None)
r = None
while r is None:
r = mydeafultdict['foo']
a defaultdict works just like a regular dictionary, except when a key doesn't exist, it calls the function supplied to it in the constructor to return a value. In this case, I gave it a lambda that just returns None. With this, you can keep trying to get foo, when there is a value associated with it, it will be returned.
I find myself in a lot of situations where I have a dictionary value that I want to update with a new value, but only if the new value fulfils some criteria relative to the current value (such as being larger).
Currently I write expressions similar to:
dictionary[key] = max(newvalue, dictionary[key])
which works fine but I keep thinking that there's probably a neater way to do it that doesn't involve repeating myself.
Thanks for any suggestions.
You could make the values objects with update methods that encapsulate that logic. Or subclass dictionary and modify the behavior of __setitem__. Just keep in mind anything you do like this is going to make it less clear to someone not familiar with your code what is going on. What you are doing now is most explicit and clear.
Just write yourself a helper function:
def update(dictionary, key, newvalue, func=max):
dictionary[key] = func(dictionary[key], newvalue)
Not sure if it's "neater", but one way to avoid repeating yourself is to use an object-oriented approach and subclass the built-in dict class to make something able to do what you want. This also has the advantage that instances of your custom class can be used in place of dict instances without changing the rest of your code.
class CmpValDict(dict):
""" dict subclass that stores values associated with each key based
on the return value of a function which allow the value passed to be
first compared to any already there (if there is no pre-existing
value, the second argument passed to the function will be None)
"""
def __init__(self, cmp=None, *args, **kwargs):
self.cmp = cmp if cmp else lambda nv,cv: nv # default returns new value
super(CmpValDict, self).__init__(*args, **kwargs)
def __setitem__(self, key, value):
super(CmpValDict, self).__setitem__(key, self.cmp(value, self.get(key)))
cvdict = CmpValDict(cmp=max)
cvdict['a'] = 43
cvdict['a'] = 17
print cvdict['a'] # 43
cvdict[43] = 'George Bush'
cvdict[43] = 'Al Gore'
print cvdict[43] # George Bush
What about using the Python version of a ternary operator:
d[key]=newval if newval>d[key] else d[key]
or a one line if:
if newval>d[key]: d[key]=newval
Consider this Python segment:
def someTestFunction():
if someTest:
return value1
elif someOtherTest:
return value2
elif yetSomeOtherTest:
return value3
return None
def SomeCallingFunction():
a = someTestFunction()
if a != None:
return a
... normal execution continues
Now, the question: the three-line segment in the beginning of SomeCallingFunction to get the value of the test function and bail out if it's not None, is repeated very often in many other functions. Three lines is too long. I want to shorten it to one. How do I do that?
I can freely restructure this code and the contents of someTestFunction however needed. I thought of using exceptions, but those don't seem to help in cutting down the calling code length.
(I've read a bit about Python decorators, but haven't used them. Would this be the place? How would it work?)
If you want to use a decorator, it would look like this:
def testDecorator(f):
def _testDecorator():
a = someTestFunction()
if a is None:
return f()
else: return a
return _testDecorator
#testDecorator
def SomeCallingFunction():
... normal execution
When the module is first imported, it runs testDecorator, passing it your original SomeCallingFunction as a parameter. A new function is returned, and that gets bound to the SomeCallingFunction name. Now, whenever you call SomeCallingFunction, it runs that other function, which does the check, and returns either a, or the result of the original SomeCallingFunction.
I often use a hash table in place of a series of elifs:
def someTestFunction(decorated_test):
options = {
'val1': return_val_1,
'val2': return_val_2
}
return options[decorated_test]
You can set up options as a defaultdict(None) to default to None if a key isn't found.
If you can't get your tests in that form, then a series of if statements might actually be the best thing to do.
One small thing you can do to shorten your code is to use this:
if a: return a
There may be other ways to shorten your code, but these are the ones I can come up with on the spot.
I think this would do it:
UPDATE Fixed!
Sorry for yesterday, I rushed and didn't test the code!
def test_decorator( test_func ):
def tester( normal_function ):
def tester_inner():
a = test_func()
if a is not None:
return a
return normal_function()
return tester_inner
return tester
#usage:
#test_decorator( my_test_function )
def my_normal_function():
#.... normal execution continue ...
It's similar to DNS's answer but allows you to specify which test function you want to use