Related
I am fairly new to intermediate programming, yet have played around with code for a while.
At the moment I am making a simple card game.
This issue is I am not sure where to use my functions or when to make them class methods.
For example, here is a simple function that deals out 5 cards to each player (from a predefined list) and then turns a card from the top of the pack (actually all just random selections).
The cards are returned as a list of items (3 lists).
I have also made a class called "Player".
p1_hand = []
p2_hand = []
flip_card = []
def deal_deck():
count = 0
for i in range(0, 10, 1):
count += 1
if count % 2 == 0:
card = random.choice(deck)
p2_hand.append(card)
deck.remove(card)
else:
card = random.choice(deck)
p1_hand.append(card)
deck.remove(card)
if count == 10:
flip_card.append(random.choice(deck))
return p2_hand, p1_hand, flip_card
In this example, it's just a deal, so I wonder why it would need to be a class method of "Player"?
Possibly the class "Player" does not do much at all except keep score, keep track of who is dealing and what the score is?
To put it simply I am having trouble understanding the class as an object that interacts and preforms actions, I have made other classes and they have ended up working like a mini database rather than using much complexity in the methods at all.
There is an art to designing classes and objects. The fundamental purpose of using classes is information hiding. The rest of your program should not have to know how a deck of cards is implemented, which allows you to change the implementation without redesigning the whole program. Thus, you create a Deck class that has all of the data stored internally, and only exposes to the outside world the things you want to DO with a deck, like shuffle and deal_card. A Player class might include a hand and a score, and functions to add another card, and the Game object (maybe) could coordinate dealing cards into the hand and triggering the plays.
The code you have is mixing all of this. It has to know how a deck is implemented, and how a hand is implemented, and how a card is flipped.
By the way, for the sake of realism, it would be better for you to shuffle the deck and deal cards off the top, instead of using random.choice.
I originally down-voted this as too broad, but changed my mind as I wrote up my notes for you. Classes are a programming tool whose implementation doesn't receive much treatment at the level you're asking. There are many examples of good card-game classes available on the Internet ... and many bad ones. The research isn't easy for you.
Use a class to represent a unit of your system (a card game, in this case) that is cohesive (a set of data and capabilities with a readily understood boundary) and interacts with other units, or with the main program.
In this case, you have a good start: you've identified card, player, and hand as entities in your game system. You may want to treat the deck as a hand instance (just another list of cards), or you may want to give it special treatment due to different functions within the game.
The classes and functions I've seen as useful include:
Deck
The impartial source of cards
data
a list of cards
methods
reset -- shuffle all 52 cards into a list
deal(n) -- return a list of n cards
Hand
cards held by a single player
data
a list of cards
methods
reset -- whatever is needed to return the hand to a game-start state
draw(n) -- acquire n cards
play(n) -- play n cards to the game area
Card
A single card, containing all information needed to identify it to the game
data
suit
rank
methods
none
Player
Game information about each player
data
hand -- see Hand class above
game-role -- depending on the game, this could be "dealer", "idle", "active", ...
... other, depending on the game: points, money, etc.
methods
... very dependent on the game being played
Game
the overall monitor for the game
data
roster -- list of players
deck -- see Deck class above
... other, depending on the game: round, player to play next, etc.
Some of these overlap a bit, such as "Deck.deal" and "Hand.draw".
One of the design decisions you face is to choose which entity will drive an interaction between two objects.
As for implementation, I do suggest that you make your basic operations a lot simpler.
For the desk, initialize by generating all 52 cards with a nested list comprehension.
shuffle the deck, and then use list slicing to deal the cards. For instance, to deal 5 cards to each player:
def deal(n):
take = deck[:n]
deck = deck[n:]
return take
# elsewhere, to drive the initial dealing ...
for player in roster:
player.hand = deck.deal(5)
Simply reduce the interactions to match the way you talk about the game: "deal each player five cards" should look just like that in the code. Bury the mechanics within each class.
So, I have looked into this question and haven't gotten what I see to be a solid answer, or maybe I just am lacking an understanding of this. Essentially I want to know:
A. Is it bad practice to have many instances of the same class?
B. What is a way to get rid of an overwhelming amount of instances without the program exiting?
Let me explain. Say I want to write a Zero Person RPG that is running in the background consistently. So I create an Enemy class for the Hero to slay.
class Enemy:
# Attr = Stats held in a dict
def __init__(self, attr={}):
self.attr = attr
A simple example. Is there an alternative to having to do the following hundreds of times?
giant = Enemy({'atk': 10, 'def': 5})
poltergeist = Enemy({'atk': 7, 'def' 8})
...
Or is this seen as the pythonic way?
No, it is not a bad practice to have many instances of the same class. When working with ORMs, you'll be working with a lot of objects, and it is totally fine as long as you are not being redundant and your use case needs the instances, and deleting the objects when you do not need them anymore.
Various ways of deleting and object are explained in this answer.
This is another answer which showcases use of with statement to manage objects contextually.
I need to make a league table for a project. There has to be 3 files,2 files consist of 1 class and the last file is for running a program. I have done all of the parts but when I call a method to add a team, the program adds the name but it does not insert it into the list of teams(which should do). When I try to display the items in the list, the program displays an error message instead of showing the actual team.
How can I fix it?Any help would be appreciated. :)
A few things here:
When I try to display the items in the list, the program displays: team.Team object at 0x000000000332A978 insted of showing the actual team.
The default display for a user class is something like <team.Team object at 0x000000000332A978>. If you want it to display something different, you have to tell Python what you want to display. There are two separate functions for this: __repr__ and __str__. The idea is that the first is a representation for the programmer, the second for the user. If you don't need two different representations, just define __repr__ and it'll use that whenever it needs __str__.
So, a really simple way to fix this is to add this to the Team class:
def __repr__(self):
return 'Team("{}")'.format(self._name)
Now, if you call league.addTeam('Dodgers'), then print(l._table), you'll get [Team("Dodgers")] instead of [<team.Team object at 0x000000000332A978>].
Meanwhile, these two methods are probably not what you want:
def removeTeam(self,team):
self._table.remove(team)
def returnPosition(self,team):
return self._table.index(team)
These will remove or find a team given the Team object—not the name, or even a new Team created from the name, but a reference to the exact same object stored in the _table. This is not all that useful, and you seem to want to call them with just names.
There are two ways to fix this: You could change Team so that it compares by name instead of by object identity, by adding this method to the class:
def __eq__(self, other):
return self._name == other._name
What this means is that if you say Team('Giants') == Team('Giants'), it will now be true instead of False. Even if the first team is in a different league, and has a different W-L record, and so on (e.g., like the baseball "Giants" from San Francisco vs. the football "Giants" from New York), as far as Python is concerned, they're now the same team. Of course if that's not what you want, you can write any other __eq__ function that seems more appropriate.
Anyway, if you do this, the index and remove functions will now be able to find any Team with the same name, instead of just the exact same team, so:
def removeTeam(self,team_name):
self._table.remove(Team(team_name))
def returnPosition(self,team_name):
return self._table.index(Team(team_name))
If you go this way, you might want to consider defining all of the comparison methods, so you can, e.g., sort a list of teams, and they sort by name.
Or you could change these methods so they don't work based on equality, e.g., by redefining them like this:
def removeTeam(self,team_name):
self._table = [team for team in self._table if team._name != team_name]
def returnPosition(self,team_name):
return [team._name for team in self._table].index(team_name)
To understand how these work, if you're not used to reading list comprehensions, turn each one back into the equivalent loop:
self._table = [team for team in self._table if team._name != team_name]
temp = []
for team in self._table:
if team._name != team_name:
temp.append(team)
self._table = temp
If you step through this, temp ends up with a list of every team in the table, except the one you wanted to remove, and then you replace the old self._table with the new filtered one. (Another way to write the same idea is with filter, if you know that function.)
It's usually better to create a new filtered list than to modify a list in-place. Sometimes there are performance reasons not do this, and sometimes it ends up being very complex and hard to understand, but it's usually both faster and simpler to reason about. Also, modifying lists in place leads to problems like this:
for i, value in enumerate(mylist):
if value == value_to_remove:
del mylist[i]
Play with this for a while, and you'll see that it doesn't actually work. Understanding why is a bit complicated, and you probably don't want to learn that until later. The usual trick to solve the problem is to iterate over a copy of the list… but once you're doing that, you've now got the worst of filtering and the worst of deleting-in-place at the same time.
The second function may be a little too clever, but let's look at it:
def returnPosition(self,team_name):
return [team._name for team in self._table].index(team_name)
First, I'm creating a list like the original one, but it's a list of just the names instead of the team objects. Again, let's decompose the list comprehension:
temp = []
for team in self._table:
temp.append(team._name)
Or try to translate it into English: This is a list of the team name of every team in the table.
Now, because this is a list of team names, I can use index(team_name) and it will find it. And, because the two lists have the same shape, I know that this is the right index to use in the original team list as well.
A much simpler solution would be to change _tables from a list of Teams into a dict mapping names to Teams. This is probably the most Pythonic solution—it looks a lot simpler than writing list comprehensions to do simple operations. (It's also probably the most efficient, but that's hardly relevant unless you have some truly gigantic leagues.) And then you don't even need returnPosition for anything. To do that:
def __init__(self):
self._table={}
def addTeam(self,name):
self._table[name]=Team(name)
def removeTeam(self,team_name):
del self._table[team_name]
def returnPosition(self,team_name):
return team_name
def updateLeague(self,team1_name1,team_name2,score1,score2):
if score1>score2:
self._table[team_name1].win()
self._table[team_name2].loss()
elif score1==score2:
self._table[team_name1].draw()
self._table[team_name2].draw()
elif score1<score2:
self._table[team_name1].loss()
self._table[team_name2].win()
Note that I've defined returnPosition to just return the team name itself as the position. If you think about it, dict keys are used exactly the same way as list indices, so this means any code someone wrote for the "old" API that required returnPosition will still work with the "new" API. (I probably wouldn't try to sell this to a teacher who assigned a problem that required us to use returnPosition, but for a real-life library where I wanted to make it easier for my 1.3 users to migrate to 2.0, I probably would.)
This only requires a few other changes. In displayList and saveList, you iterate over self._table.values() rather than self._table; in loadList, you change self._table.append(team) to self._table[a] = team. Speaking of loadList: You might want to consider renaming those local variables from a, b, c, and d to name, wins, losses, and draws.
A few other comments:
As kreativitea says in the comments, you should not create "private" variables and then add do-nothing accessor methods in Python. It's just more boilerplate that hides the real code, and one more thing you can get wrong with a silly typo that you'll spend hours debugging one day. Just have members named name, wins, losses, etc., and access them directly. (If someone told you that this is bad style because it doesn't let you replace the implementation in the future without changing the interface, that's only true in Java and C++, not in Python. If you ever need to replace the implementation, just read up on #property.)
You don't need print("""""")—and it's very easy to accidentally miscount the number of " characters. (Especially since some IDEs will actually be confused by this and think the multi-line string never ends.) Just do print().
You've got the same ending condition both in the while loop (while x!="q":) and in an internal break. You don't need it in both places. Either change it to while True:, or get rid of the break (just make options("q") do print("Goodbye"), so you don't need to special-case it at all inside the loop).
Whenever you have a long chain of elif statements, think about whether you can turn it into a dict of short functions. I'm not sure it's a good idea in this case, but it's always worth thinking about and making the explicit decision.
The last idea would look something like this:
def addTeam():
name=input("Enter the name of the team:")
l.addTeam(name)
def removeTeam():
teamToRemove=input("Enter the name of the team you want to remove:")
l.removeTeam(teamToRemove)
def recordGame():
team1=input("What is the name of the team?")
ans1=int(input("Enter the number of goals for the first team:"))
team2=input("What is the name of the team?")
ans2=int(input("Enter the number of goals for the second time:"))
l.updateLeague(team1,team2,ans1,ans2)
optionsdict = {
"a": addTeam,
"d": l.displayList,
"s": l.saveList,
"l": l.loadList,
"r": removeTeam,
"rec": recordGame,
}
def options(x):
func = optionsdict.get(x)
if func:
func()
As I said, I'm not sure it's actually clearer in this case, but it's worth considering.
I've made two classes called House and Window. I then made a list containing four Houses. Each instance of House has a list of Windows. I'm trying to iterate over the windows in each house and print it's ID. However, I seem to get some odd results :S I'd greatly appreciate any help.
#!/usr/bin/env python
# Minimal house class
class House:
ID = ""
window_list = []
# Minimal window class
class Window:
ID = ""
# List of houses
house_list = []
# Number of windows to build into each of the four houses
windows_per_house = [1, 3, 2, 1]
# Build the houses
for new_house in range(0, len(windows_per_house)):
# Append the new house to the house list
house_list.append(House())
# Give the new house an ID
house_list[new_house].ID = str(new_house)
# For each new house build some windows
for new_window in range(0, windows_per_house[new_house]):
# Append window to house's window list
house_list[new_house].window_list.append(Window())
# Give the window an ID
house_list[new_house].window_list[new_window].ID = str(new_window)
#Iterate through the windows of each house, printing house and window IDs.
for house in house_list:
print "House: " + house.ID
for window in house.window_list:
print " Window: " + window.ID
####################
# Desired output:
#
# House: 0
# Window: 0
# House: 1
# Window: 0
# Window: 1
# Window: 2
# House: 2
# Window: 0
# Window: 1
# House: 3
# Window: 0
####################
Currently you are using class attributes instead of instance attributes. Try changing your class definitions to the following:
class House:
def __init__(self):
self.ID = ""
self.window_list = []
class Window:
def __init__(self):
self.ID = ""
The way your code is now all instances of House are sharing the same window_list.
Here's the updated code.
# Minimal house class
class House:
def __init__(self, id):
self.ID = id
self.window_list = []
# Minimal window class
class Window:
ID = ""
# List of houses
house_list = []
# Number of windows to build into each of the for houses
windows_per_house = [1, 3, 2, 1]
# Build the houses
for new_house in range(len(windows_per_house)):
# Append the new house to the house list
house_list.append(House(str(new_house)))
# For each new house build some windows
for new_window in range(windows_per_house[new_house]):
# Append window to house's window list
house_list[new_house].window_list.append(Window())
# Give the window an ID
house_list[new_house].window_list[new_window].ID = str(new_window)
#Iterate through the windows of each house, printing house and window IDs.
for house in house_list:
print "House: " + house.ID
for window in house.window_list:
print " Window: " + window.ID
The actual problem is that the window_list attribute is mutable, so when the different instances are using it, they end up sharing the same one. By moving window_list into __init__ each instance gets its own.
C++, Java, C# etc. have this really strange behaviour regarding instance variables, whereby data (members, or fields, depending on which culture you belong to) that's described within a class {} block belongs to instances, while functions (well, methods, but C++ programmers seem to hate that term and say "member functions" instead) described within the same block belong to the class itself. Strange, and confusing, when you actually think about it.
A lot of people don't think about it; they just accept it and move on. But it actually causes confusion for a lot of beginners, who assume that everything within the block belongs to the instances. This leads to bizarre (to experienced programmers) questions and concerns about the per-instance overhead of these methods, and trouble wrapping their heads around the whole "vtable" implementation concept. (Of course, it's mostly the teachers' collective fault for failing to explain that vtables are just one implementation, and for failing to make clear distinctions between classes and instances in the first place.)
Python doesn't have this confusion. Since in Python, functions (including methods) are objects, it would be bizarrely inconsistent for the compiler to make a distinction like that. So, what happens in Python is what you should intuitively expect: everything within the class indented block belongs to the class itself. And, yes, Python classes are themselves objects as well (which gives a place to put those class attributes), and you don't have to jump through standard library hoops to use them reflectively. (The absence of manifest typing is quite liberating here.)
So how, I hear you protest, do we actually add any data to the instances? Well, by default, Python doesn't restrict you from adding anything to any instance. It doesn't even require you to make different instances of the same class contain the same attributes. And it certainly doesn't pre-allocate a single block of memory to contain all the object's attributes. (It would only be able to contain references, anyway, given that Python is a pure reference-semantics language, with no C# style value types or Java style primitives.)
But obviously, it's a good idea to do things that way, so the usual convention is "add all the data at the time that the instance is constructed, and then don't add any more (or delete any) attributes".
"When it's constructed"? Python doesn't really have constructors in the C++/Java/C# sense, because this absence of "reserved space" means there's no real benefit to considering "initialization" as a separate task from ordinary assignment - except of course the benefit of initialization being something that automatically happens to a new object.
So, in Python, our closest equivalent is the magic __init__ method that is automatically called upon newly-created instances of the class. (There is another magic method called __new__, which behaves more like a constructor, in the sense that it's responsible for the actual creation of the object. However, in nearly every case we just want to delegate to the base object __new__, which calls some built-in logic to basically give us a little pointer-ball that can serve as an object, and point it to a class definition. So there's no real point in worrying about __new__ in almost every case. It's really more analogous to overloading the operator new for a class in C++.) In the body of this method (there are no C++-style initialization lists, because there is no pre-reserved data to initialize), we set initial values for attributes (and possibly do other work), based on the parameters we're given.
Now, if we want to be a little bit neater about things, or efficiency is a real concern, there is another trick up our sleeves: we can use the magic __slots__ attribute of the class to specify class attribute names. This is a list of strings, nothing fancy. However, this still doesn't pre-initialize anything; an instance doesn't have an attribute until you assign it. This just prevents you from adding attributes with other names. You can even still delete attributes from an object whose class has specified __slots__. All that happens is that the instances are given a different internal structure, to optimize memory usage and attribute lookup.
The __slots__ usage requires that we derive from the built-in object type, which we should do anyway (although we aren't required in Python 2.x, this is intended only for backwards-compatibility purposes).
Ok, so now we can make the code work. But how do we make it right for Python?
First off, just as with any other language, constantly commenting to explain already-self-explanatory things is a bad idea. It distracts the user, and doesn't really help you as a learner of the language, either. You're supposed to know what a class definition looks like, and if you need a comment to tell you that a class definition is a class definition, then reading the code comments isn't the kind of help you need.
With this whole "duck typing" thing, it's poor form to include data type names in variable (or attribute) names. You're probably protesting, "but how am I supposed to keep track of the type otherwise, without the manifest type declaration"? Don't. The code that uses your list of windows doesn't care that your list of windows is a list of windows. It just cares that it can iterate over the list of windows, and thus obtain values that can be used in certain ways that are associated with windows. That's how duck typing works: stop thinking about what the object is, and worry about what it can do.
You'll notice in the code below that I put the string conversion code into the House and Window constructors themselves. This serves as a primitive form of type-checking, and also makes sure that we can't forget to do the conversion. If someone tries to create a House with an ID that can't even be converted to a string, then it will raise an exception. Easier to ask for forgiveness than permission, after all. (Note that you actually have to go out of your way a bit in Python to create
As for the actual iteration... in Python, we iterate by actually iterating over the objects in a container. Java and C# have this concept as well, and you can get at it with the C++ standard library too (although a lot of people don't bother). We don't iterate over indices, because it's a useless and distracting indirection. We don't need to number our "windows_per_house" values in order to use them; we just need to look at each value in turn.
How about the ID numbers, I hear you ask? Simple. Python provides us with a function called 'enumerate', which gives us (index, element) pairs given an input sequence of elements). It's clean, it lets us be explicit about our need for the index to solve the problem (and the purpose of the index), and it's a built-in that doesn't need to be interpreted like the rest of the Python code, so it doesn't incur all that much overhead. (When memory is a concern, it's possible to use a lazy-evaluation version instead.)
But even then, iterating to create each house, and then manually appending each one to an initially-empty list, is too low-level. Python knows how to construct a list of values; we don't need to tell it how. (And as a bonus, we typically get better performance by letting it do that part itself, since the actual looping logic can now be done internally, in native C.) We instead describe what we want in the list, with a list comprehension. We don't have to walk through the steps of "take each window-count in turn, make the corresponding house, and add it to the list", because we can say "a list of houses with the corresponding window-count for each window-count in this input list" directly. That's arguably clunkier in English, but much cleaner in a programming language like Python, because you can skip a bunch of the little words, and you don't have to expend effort to describe the initial list, or the act of appending the finished houses to the list. You don't describe the process at all, just the result. Made-to-order.
Finally, as a general programming concept, it makes sense, whenever possible, to delay the construction of an object until we have everything ready that's needed for that object's existence. "Two-phase construction" is ugly. So we make the windows for a house first, and then the house (using those windows). With list comprehensions, this is simple: we just nest the list comprehensions.
class House(object):
__slots__ = ['ID', 'windows']
def __init__(self, id, windows):
self.ID = str(id)
self.windows = windows
class Window(object):
__slots__ = ['ID']
def __init__(self, id):
self.ID = str(id)
windows_per_house = [1, 3, 2, 1]
# Build the houses.
houses = [
House(house_id, [Window(window_id) for window_id in range(window_count)])
for house_id, window_count in enumerate(windows_per_house)
]
# See how elegant the list comprehensions are?
# If you didn't quite follow the logic there, please try **not**
# to imagine the implicitly-defined process as you trace through it.
# (Pink elephants, I know, I know.) Just understand what is described.
# And now we can iterate and print just as before.
for house in houses:
print "House: " + house.ID
for window in house.windows:
print " Window: " + window.ID
Apart from some indentation errors, you're assigning the IDs and window_lists to the class and not the instances.
You want something like
class House():
def __init__(self, ID):
self.ID = ID
self.window_list = []
etc.
Then, you can do house_list.append(House(str(newHouse))) and so on.
I'm coding a poker hand evaluator as my first programming project. I've made it through three classes, each of which accomplishes its narrowly-defined task very well:
HandRange = a string-like object (e.g. "AA"). getHands() returns a list of tuples for each specific hand within the string:
[(Ad,Ac),(Ad,Ah),(Ad,As),(Ac,Ah),(Ac,As),(Ah,As)]
Translation = a dictionary that maps the return list from getHands to values that are useful for a given evaluator (yes, this can probably be refactored into another class).
{'As':52, 'Ad':51, ...}
Evaluator = takes a list from HandRange (as translated by Translator), enumerates all possible hand matchups and provides win % for each.
My question: what should my "domain" class for using all these classes look like, given that I may want to connect to it via either a shell UI or a GUI? Right now, it looks like an assembly line process:
user_input = HandRange()
x = Translation.translateList(user_input)
y = Evaluator.getEquities(x)
This smells funny in that it feels like it's procedural when I ought to be using OO.
In a more general way: if I've spent so much time ensuring that my classes are well defined, narrowly focused, orthogonal, whatever ... how do I actually manage work flow in my program when I need to use all of them in a row?
Thanks,
Mike
Don't make a fetish of object orientation -- Python supports multiple paradigms, after all! Think of your user-defined types, AKA classes, as building blocks that gradually give you a "language" that's closer to your domain rather than to general purpose language / library primitives.
At some point you'll want to code "verbs" (actions) that use your building blocks to perform something (under command from whatever interface you'll supply -- command line, RPC, web, GUI, ...) -- and those may be module-level functions as well as methods within some encompassing class. You'll surely want a class if you need multiple instances, and most likely also if the actions involve updating "state" (instance variables of a class being much nicer than globals) or if inheritance and/or polomorphism come into play; but, there is no a priori reason to prefer classes to functions otherwise.
If you find yourself writing static methods, yearning for a singleton (or Borg) design pattern, writing a class with no state (just methods) -- these are all "code smells" that should prompt you to check whether you really need a class for that subset of your code, or rather whether you may be overcomplicating things and should use a module with functions for that part of your code. (Sometimes after due consideration you'll unearth some different reason for preferring a class, and that's allright too, but the point is, don't just pick a class over a module w/functions "by reflex", without critically thinking about it!).
You could create a Poker class that ties these all together and intialize all of that stuff in the __init__() method:
class Poker(object):
def __init__(self, user_input=HandRange()):
self.user_input = user_input
self.translation = Translation.translateList(user_input)
self.evaluator = Evaluator.getEquities(x)
# and so on...
p = Poker()
# etc, etc...