辅导data语言、Python程序设计辅导

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Make sure to set up a dedicated python environment for this project.
You can either use anaconda or venv to create a dedicated environment.
With anaconda:
conda create -n cw3 python=3.7 anaconda
conda activate cw3
# work work work
conda deactivate
with venv:
python -m venv cw4env
source ./cw3env/bin/activate
# work work work
deactivate
In your base repo:
git checkout lab3
./refresh.sh
Install the requirements
pip install -r requirements.txt
Familiarise yourself with the code. in the directory, you will find the sub-directory learning
which contains the source code (stubs). You will implement the decision tree learner in
learning/attr_learner.py and FOIL in learning/rule_learner.py . The files contain a skeleton
of the ready algorithm with method and function stubs that you will need to implement. Do not
change their signature, as tests found in the tests/ directory will be used to evaluate your
solutions automatically. Obviously, also do not change the tests.
You will have to implement your solutions against public and hidden test cases. Public solutions
are available to you and you can look at them in the folder tests/public/ . Hidden solutions
test your implementation further, for some interesting input configurations or edge cases and
Coursework 3
Step 0: Setup
Step 1: Obtain the Code
Step 2: Run the tests
are not available to you. To run all tests, run
pytest
To run all tests in a file, e.g. tests/public/test_attr_learner.py , run
pytest tests/public/test_attr_learner.py
To run a specific test case in the file, e.g. test_overall_public run
pytest tests/public/test_attr_learner.py::test_overall_public
Pytest captures your console output by default. If you don't want that, you can append the --
capture=no flag
pytest --capture=no [...]
And if you - as you should - use the python logging stdlib instead of print , you can even
control the granularity of messages pytest will let through with
pytest --log-cli-level=LEVEL [...]
where LEVEL is DEBUG , INFO , WARN , etc.
In this exercise, you will implement the Decision Tree Learner as presented in the lecture. The
skeleton is located in learning/attr_learner.py . Tests for this exercise can be found in
tests/public/test_attr_learner.py It will take a dataset of examples that are represented by
a combination of values of a fixed set of attributes and produce a learned hypothesis of the
following format:
ATTR_1_NAME = ATTR_1_VAL_1 AND ... AND ATTR_N_NAME = ATTR_N_VAL_K
OR
...
OR
ATTR_1_NAME = ATTR_1_VAL_N AND ... AND ATTR_N_NAME = ATTR_M_VAL_K
The dataclasses AttrLogicExpression , Conjunction and Disjunction represent the
hypotheses above in python code.
If you are unfamiliar with dataclasses, click here for a quick refresher.
AttrLogicExpression is an abstract base class, representing a logic expression. Conjunction is
a conjunction of specific attribute values. the field attribute_confs takes a dict mapping
from attribute names to their values. In other words a Conjunction c will be true for an
Exercise 1: Decision tree learning [7pt]
example e iff
all(c.attr_values[x] == e[x]
for x in k in set(self.attribute_confs).intersection(example))
This functionality is already implemented. You can test this by calling the conjunction, like so:
c(example)
Similar is the case with disjunctions, a Disjunction d has a field conjunctions which is a list
of conjunctions. A Disjunction d is true for an example e if all its conjunctions are true.
Similarly, you can test this with d(e) .
Other relevant dataclasses are the abstract base class Tree representing a Decision Tree
element, Node representing a decision and Leaf representing a classification. More details
can be seen in the source code documentation.
Finally, type abbreviations used are Example = Dict[str, Any] which means that one single
example is a dict mapping from attribute names to their values and Examples =
List[Example] represents a list of examples.
A special attribute is target which represents the classification of the example. For now, you
can assume that target is boolean, all attributes are the same for all Examples and all values
are given (no NULL values) for all examples.
Like you have seen in the lecture, the termination cases of the learning algorithm include
checking whether all remaining examples have the same classification and what the majority
value is. For the former, you should implement the function same_target(examples: Examples) -
> bool that takes a list of examples and returns True if they have the same target attribute.
For the latter you should implement the function plurality_value(examples: Examples) ->
bool , which returns the target value that appears most in the given dataset.
The tests test_same_target_public and test_plurality_value_public should succeed now.
Now you should implement the binary entropy as shown in the lecture, in the function
binary_entropy(examples: Examples) -> float . The function will take Examples and return the
binary entropy with regard to the classification.
Watch out for the edge cases!
The test test_binary_entropy_public should succeed now.
With binary entropy implemented, you can implement the information gain now. The method is
Implementing the termination [1p]
Implementing Binary Entropy [1p]
Implementing information gain [2p]
DecisionTreeLearner.information_gain(self, examples: Examples, attribute: str) -> float.
Like in the lecture videos, the method should take a dataset examples and an attribute name
attribute , and compute the information gain by splitting the dataset on that attribute. With
the information gain, you are also be able to implement
DecisionTreeLearner.get_most_important_attribute(self, attributes: List[str], examples:
Examples) -> str: that takes a list of attribute names and a dataset and returns the attribute
with the highest information gain
Watch out for edge cases here as well!
The following tests should succeed now:
test_information_gain_public
test_get_most_important_attribute_public
In the lecture we mentioned that a decision tree can be converted to an equivalent logic
expression. Implement the function to_logic_expression(tree: Tree) -> AttrLogicExpression
that takes a decision tree tree and converts it into an AttrLogicExpression which, as
described above is a disjunction of conjunctions of attribute configurations. Note that the
conjunctions should be ordered such that the topmost attribute in a path in the decision tree
should appear leftmost in the conjunction. The order of the disjunctions is irrelevant. Again,
you won't need to do the string conversion, just represent the tree as a combination of
Conjunction and Disjunction python objects, they already implement the corresponding
__str__ methods to convert them a string of the form shown above.
Now you're equipped to bring the pieces together and implement the full algorithm as
presented in the lecture.
The recursive function decision_tree_learning(self, examples: Examples, attributes:
List[str], parent_examples: Examples) -> Tree should be implemented to take a dataset, a list
of attributes that describe examples of that dataset and all examples from the parent decision
step and return an induced decision tree.
Make sure to watch out for edge cases and implement all terminating cases correctly!
The test test_overall_public should succeed now.
In this exercise, we will make use of pyswip, a bridge between Python and Prolog. Yes, we're
going to call Prolog from Python. Make sure, that swipl is accessible from your path by testing
whether you can run swipl successfully in your environment.
The overall layout will be the following: given a kb from a text file and positive and negative
examples of the target relation to be learned, the goal is to learn the relation as a disjunction
Implementing Tree to Logic Expression [2p]
Implementing The full algorithm [1p]
Exercise 2: FOIL [9p]
of horn clauses.
The skeleton implementation as well as the Python objects representing elements of Prolog can
be found in learning/rule_learner.py . Tests are located in
tests/public/test_rule_learner.py .
Take a look at the documentation of the Python logic objects to understand how they are
working.
Note: The pyswip bridge can be wonky at times, not clearing the knowledge base between
test runs. I recommend to run tests involving the KB in isolation rather than all together. This is
especially relevant for the last two tasks in this exercise.
Implement _FOIL.get_predicates(self) -> List[Predicate] which will extract all relevant
predicates from the kb.
Hint 1: to access Prolog from within get_predicates , use self.prolog
Hint 2: to access the KB file path, use self.dataset.kb .
Hint 3: Prolog's built-in predicate predicate_property/2 might come in handy! Test it and pay
attention to the second argument. How can you make use of it to effectively filter for relevant
predicates?
The test test_get_predicates_public should succeed now.
To check whether an example is covered by a Horn clause, implement covers(self, clause:
HornClause, example: Example) -> bool . This will take a horn clause and an example and return
true if the example is covered by the clause. Similar to above, you can make use of Prolog
here, to make your life easier.
The test test_rule_covers_public should succeed now.
Just as discussed in the lecture, the function
_FOIL.generate_candidates(self,
clause: HornClause,
predicates: List[Predicate]
) -> Generator[Expression, None, None]
shall take a horn clause and return a python generator that contains all (reasonable) literals
which specialise the clause. Follow the lecture video, but for now, don't worry about negation
or any of the other more advanced use cases such as recursion, (dis)equality or arithmetics.
When introducing literals with new variables, make sure that the variables are named
Get predicates from the KB [1p]
Implement covers [1p]
Generate Candidates [2p]
uniquely! Implement the function _FOIL.unique_var(self) to return the next unique variable
every time it's called. For simplicity (and to comply with the test cases later), the variables
should be named V_i where i is an integer starting from 0 and increasing by 1 with every
subsequent call.
The test test_generate_candidates should succeed now.
Note on python generators: If you don't feel comfortable using generators it's also okay to
return lists. The tests should not fail because of this. You might just get annoying type
checking warnings if you use an IDE/editor with type checking support.
To extend an example with all valid substitutions according to the knowledge base and subject
to a given expression, as seen in the lecture videos, implement extend_example(self, example:
Example, new_expr: Expression) -> Generator[Example, None, None] . This function shall take an
example and an expression, and return a python generator of examples (that means
potentially more than one) that represent a valid substitution. See lecture video for an
example.
Hint: Use Prolog here. Think about how to formulate a Prolog query to get the substitutions.
The test test_extend_example_public should pass now.
Now you're in the position to implement the method that guides the search for the
specialisations: _FOIL.foil_information_gain(self, candidate: Expression, pos_ex: Examples,
neg_ex: Examples) -> float . This method shall take a candidate literal (as generated by
generate_candidates ), the dataset consisting of positive and negative examples and evaluate
the information gain from the candidate. See lecture video for an example.
Make sure to implement is_represented_by(example: Example, examples: Examples) -> bool to
calculate the factor t of the FOIL information gain formula.
Once again, pay attention to edge cases!
The tests test_foil_ig_public and test_is_represented_by_public should succeed now.
Now you're equipped to put everything together. Implement
_FOIL.foil(self,
positive_examples: Examples,
negative_examples: Examples,
predicates: List[Predicate],
target: Literal) -> List[HornClause]
which shall be the outer loop of the FOIL algorithm and
Extending examples [1p]
Foil information gain [2p]
Putting it all together [1p]
_FOIL.new_clause(self,
positive_examples: Examples,
negative_examples: Examples,
predicates: List[Predicate],
target: Literal) -> HornClause
which shall be the inner loop of the algorithm according to the pseudo-code presented in the
lecture.
Make sure that your code terminates!
The following tests should succeed:
test_new_clause_public
test_overall_public
test_overall_public_2
Our FOIL algorithm isn't quite good so far. Make it learn recursive relations!
Hint: you will want to modify which predicates are allowed at which point.
The test test_recursive_public should run now. (It takes a while)
Well done, if you've arrived here, all your tests should succeed.
However, the implemented algorithms so far are pretty basic. It's time to make them better!
You can choose one (or multiple) algorithms and improve them along the following metrics:
Accuracy with noisy data
Accuracy with less training examples
Training time
Alternatively, you can make conceptual improvements to the algorithms (e.g. such as
introducing recursion in the example above). In that case you should also provide a
corresponding dataset where the base algorithm fails, but your implementation succeeds.
Make sure to provide explanations for what you did in a separate Exercise3.md file.
Note that in order to get full marks here, you will need to implement multiple "impressive"
improvements. Since FOIL can be seen as conceptually more challenging, the improvements
on FOIL will be seen as more "impressive".
The functionality to compare algorithms along the metrics and to generate datasets is
provided. To find out how to use it and how to implement your own improved algorithms, make
sure to check out the video explaining the course work and readme-scripts.MD .
Make sure to document well what you have done! Try to describe what you did, why you did it
what you hypothesise will happen and whether experiments confirm or reject your hypothesis.
Recursive case [1p]
Exercise 3: Make it better [4p]
Note that a well documented failure will probably give you a better score than a "mystic
success".
Submission follows the usual mechanism. Add and commit your code, then and tag it with the
./submit.sh command. For exercise 1 and 2 you will need submit your (documented) code.
For exercise 3, you will need to submit your code as well as a readme labelled Exercise3.md
that describes the changes you made, how they improve upon the algorithms, and the
commands to replicate your evidence in support of your arguments, i.e: what do the markers
need to run to see the improvements for themselves?
The weight of the coursework for the final course grade is equal to that of the previous
assignments.
The rationale behind the marking scheme is the following: The test cases released to you
represent a bare minimum example of the intended functionality that you need to implement.
Further hidden test cases might check for additional interesting scenarios where the
implemented methods/functions are supposed to work, for example some edge cases. These
will not be released, but will be taken into consideration when determining the final grade.
Feedback will be provided to you based on whether the tests succeed or fail. Furthermore,
you're expected to understand the functionality of your code as evidenced by an appropriate
documentation of the functionality/comments in the code. This results in the following marking
scheme for all Exercises 1 and 2:
Grade Explanation
0 No reasonable attempt to solve the exercise has been made
0.25 A reasonable attempt has been made, but public tests fail
0.5 Public tests succeed but all hidden tests fail
0.75 Public tests succeed and some hidden test fails
1 All (public+hidden) tests succeed
If the code is not sufficiently documented, the overall grade will be reduced up to 1p per
exercise at the discretion of the markers.
Example for insufficient explanation/documentation:
def some_func(text):
text = str(text)
# get the name
name = text[:text.find("(")].strip()
# get the arity
arity = len(re.findall("Variable", text))
# if arity zero
if arity == 0:
Submission
Marking scheme
# find arity
arity = len(re.findall(",", text)) + 1
# return predicate
return Predicate(name, arity)
Example for exhaustive explanation/documentation (Note that you should aim for sufficient
rather than exhaustive):
def some_func(text):
# assuming input is a single predicate with its
# arguments in valid prolog syntax
text = str(text)
# then name of predicate comes before
# the first bracket in expression
name = text[:text.find("(")].strip()
# and arity is either defined by number
# of Variable in expression
# (as returned by predicate_property/2)
arity = len(re.findall("Variable", text))
# or, if not result of that query
if arity == 0:
# then we just count the commas that separate
# the arguments
arity = len(re.findall(",", text)) + 1
return Predicate(name, arity)
Note that implementing a function/method to directly (and only) to return the expected test
value will automatically and inevitably result in 0 points for that exercise. To illustrate, the
following test/implementation example will give 0 marks although the test is passing:
def test_foo():
assert foo("bar") == 5
def foo(arg):
return 5
Exercises worth more than one mark are scaled accordingly.
For Exercise 3, the mile stones for marks are set as follows:
Grade Explanation
0 No reasonable attempt to improve any of the algorithms has been made
1
A reasonable attempt to improve one of the algorithms has been made, but
without any visible impact
2
A good attempt to improve one of the algorithms has been made, the
improvements are visible
3
An impressive improvement to one the algorithms have been made, the
improvements are visible and well documented. Alternatively, a new algorithm
has been implemented and its advantages have been demonstrated clearly
4 Like above but more than one improvement/algorithm