======Spam filter - step 1======
We are going to create a function, which can read the information from files ''!truth.txt'' or ''!prediction.txt'' into the //dictionary// data structure.

/**
<WRAP round download>
  * [[.unit_testing|Tests]] for step 1: {{:courses:a4b99rph:cviceni:spam:test1_readclassification.zip|}}
</WRAP>
**/

=====Preparation=====

++++ Working with a dictionary |
  * See {[a4b99rph:Pilgrim2009]}, chapter [[http://www.diveinto.org/python3/native-datatypes.html#dictionaries|2.7]], or {[a4b99rph:Wentworth2012]}, chapter [[http://openbookproject.net/thinkcs/python/english3e/dictionaries.html|20]]).
  * How to create an empty dictionary.
  * How to add a key-value pair.
  * How to read a value of a key.
  * How to browse the dictionary by items using method ''items()'':<code python>
eng_to_cz = {'cat': 'kocka', 'dog': 'pes', 'house': 'dum' }
for eng, cz in eng_to_cz.items():
    print(eng, ',', cz)
</code>
++++

++++ Working with (text) files |
  *(viz {[a4b99rph:Pilgrim2009]}, chapter [[http://diveinto.org/python3/files.html|11]], or {[a4b99rph:Wentworth2012]}, chapter [[http://openbookproject.net/thinkcs/python/english3e/files.html|13]]).
  * How to open and close a text file.
  * How to use the ''with'' command.
  * Reading a file line by line.
  * Reading the whole file contents as a single string.
++++

++++ The usage of section ''if __name__ == "__main__":'' |
  * See {[a4b99rph:Pilgrim2009]}, chapter [[http://diveinto.org/python3/your-first-python-program.html#runningscripts|1.10]]).
++++

++++ Method ''split()'' of string values |
  * See the Python docs for [[http://docs.python.org/py3k/library/stdtypes.html?highlight=split#str.split|str.split()]])
++++


===== Reading classification from a file =====

Task:
  * In a module called ''utils.py'', create a function ''read_classification_from_file()'' that will read the mail classes from a text file.

Why do we need it:
  * We will need this function if we want to create a learning filter, and during the evaluation of the filter quality. 

==== Specifications ====
Function ''read_classification_from_file()'' (in module ''utils.py'') has to conform to the following specifications: 

^ ''read_classification_from_file(fpath)'' ^^
^  Input  | The path to the text file (most likely either ''!truth.txt'' or ''!prediction.txt'')  |
^  Output  | A dictionary containing either ''SPAM'' or ''OK'' label for each filename in email corpus.  |

The function loads a text file contaning a pair of strings per line, separated by single space, like this:
<code>
email01 OK
email02 OK
email03 SPAM
email1234 OK
</code>
and creates a dictionary (the order of individual "rows" in the following listing is not important):
<code python>
{'email1234': 'OK', 'email03': 'SPAM', 'email02': 'OK', 'email01': 'OK'}
</code>

If the file is empty, it returns an empty dictionary.


> {{page>courses:a4b99rph:internal:cviceni:spam:tyden07#read_classification_from_file&editbtn}}


===== Writing classification (predictions) to a file =====

Task:
  * In module ''utils.py'', create function ''write_classification_to_file()'' that will write the (usually predicted) mail classes to a text file.

Why do we need it:
  * The function will come handy when writing the filter; it can be used to create the ''!prediction.txt'' file. 

==== Specifications ====
Function ''write_classification_to_file()'' (in module ''utils.py'') should conform to the following specifications: 

^ ''write_classification_to_file(cls_dict, fpath)'' ^^
^  Inputs  | (1) dictionary containing the email file names as keys, and email classes (''SPAM'' or ''OK'') as values. |
^          | (2) The path to the text file that shall be created.  |
^  Output  | None.  |

The following code

<code python>
>>> cls_dict = {'email1234': 'OK', 'email03': 'SPAM', 'email02': 'OK', 'email01': 'OK'}
>>> fpath = '1/!prediction.txt'
>>> write_classification_to_file(cls_dict, fpath)
</code>

shall create file ''!prediction.txt'' in directory ''1'' (the directory must exist) with the following contents:

<code>
email01 OK
email02 OK
email03 SPAM
email1234 OK
</code>

The actual order of individual rows in the file is not important.

If the ''cls_dict'' is empty, the function shall create an empty file.