Problems with dictionaries

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Problems with dictionaries

 
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  #1
Nov 8th, 2009
Assume I have a file of the following format:
a,1
b,2
c,3
d,4
Here is my code:
Python Syntax (Toggle Plain Text) 
  1. def junk(f):
  2.     d1 = {}
  3.     d2 = {}
  4.     for line in f:
  5.         columns = line.split(":")
  6.         letters = columns[1]
  7.         numbers = columns[2]
  8.         d1[letters] = numbers
  9.         d2[numbers] = letters
  10.     return (d1, d2)
  11.  
  12.  
  13. def something():  
  14.     print d1
  15.     print d2
  16.  
  17. if __name__ == "__main__":
  18.     f = open("filename.txt")
  19.     d1 = junk(f)[0]
  20.     d2 = junk(f)[1]

Assume I want to call function something. It cannot print d1 and d2 unless I add them to the main block. As soon as I add d1 and d2 to the "main" block and call either of the two functions, both return two empty dictionaries. Why does this happen? How can I fix it? Please help!
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  #2
Nov 8th, 2009
First you are not returning a dictionary. I've added a print statement to show that. Also, take a look at "returns" and "arguments" here http://www.penzilla.net/tutorials/python/functions/ for the "something" function.
Python Syntax (Toggle Plain Text) 
  1. def junk(f):
  2.     d1 = {}
  3.     d2 = {}
  4.     for line in f:
  5.         columns = line.split(":")
  6.         letters = columns[1]
  7.         numbers = columns[2]
  8.         d1[letters] = numbers
  9.         d2[numbers] = letters
  10.     return (d1, d2)
  11.  
  12.  
  13. def something():  
  14.     print d1
  15.     print d2
  16.  
  17. if __name__ == "__main__":
  18.     f = open("filename.txt")
  19.     d1 = junk(f)[0]
  20.     print "type d1 =", type(d1)
  21.     d2 = junk(f)[1]
Last edited by woooee; Nov 8th, 2009 at 1:19 pm.
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  #3
Nov 8th, 2009
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Originally Posted by woooee View Post
First you are not returning a dictionary. I've added a print statement to show that. Also, take a look at "returns" and "arguments" here http://www.penzilla.net/tutorials/python/functions/ for the "something" function.
Python Syntax (Toggle Plain Text) 
  1. def junk(f):
  2.     d1 = {}
  3.     d2 = {}
  4.     for line in f:
  5.         columns = line.split(":")
  6.         letters = columns[1]
  7.         numbers = columns[2]
  8.         d1[letters] = numbers
  9.         d2[numbers] = letters
  10.     return (d1, d2)
  11.  
  12.  
  13. def something():  
  14.     print d1
  15.     print d2
  16.  
  17. if __name__ == "__main__":
  18.     f = open("filename.txt")
  19.     d1 = junk(f)[0]
  20.     print "type d1 =", type(d1)
  21.     d2 = junk(f)[1]
I still don't get it. The type of d1 is dictionary.
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  #4
Nov 8th, 2009
Try this and then go to the page from the earlier link which explains why you returned a tuple.
Python Syntax (Toggle Plain Text) 
  1. def junk(f):
  2.     d1 = {}
  3.     d2 = {}
  4.     for line in f:
  5.         columns = line.split(":")
  6.         letters = columns[1]
  7.         numbers = columns[2]
  8.         d1[letters] = numbers
  9.         d2[numbers] = letters
  10.     return d1, d2
  11.  
  12. if __name__ == "__main__":
  13.     f = open("filename.txt")
  14.     d1_ret, d2_ret = junk(f)[0]
  15.     print type(d1_ret)
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  #5
Nov 8th, 2009
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Originally Posted by woooee View Post
Try this and then go to the page from the earlier link which explains why you returned a tuple.
Python Syntax (Toggle Plain Text) 
  1. def junk(f):
  2.     d1 = {}
  3.     d2 = {}
  4.     for line in f:
  5.         columns = line.split(":")
  6.         letters = columns[1]
  7.         numbers = columns[2]
  8.         d1[letters] = numbers
  9.         d2[numbers] = letters
  10.     return d1, d2
  11.  
  12. if __name__ == "__main__":
  13.     f = open("filename.txt")
  14.     d1_ret, d2_ret = junk(f)[0]
  15.     print type(d1_ret)
OK, I see that putting a comma between two variables automatically returns a tuple. But the problem is that as long as d1(d1_ret) or d2(d2_ret) in the "main" block junk(f) would return twoempty dictionaries in a tuple. That is what I don't understand.
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  #6
Nov 8th, 2009
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Originally Posted by pyprog View Post
Assume I have a file of the following format:
a,1
b,2
c,3
d,4
Here is my code:
Python Syntax (Toggle Plain Text) 
  1. def junk(f):
  2.     d1 = {}
  3.     d2 = {}
  4.     for line in f:
  5.         columns = line.split(":")
  6.         letters = columns[1]
  7.         numbers = columns[2]
  8.         d1[letters] = numbers
  9.         d2[numbers] = letters
  10.     return (d1, d2)
  11.  
  12.  
  13. def something():  
  14.     print d1
  15.     print d2
  16.  
  17. if __name__ == "__main__":
  18.     f = open("filename.txt")
  19.     d1 = junk(f)[0]
  20.     d2 = junk(f)[1]

Assume I want to call function something. It cannot print d1 and d2 unless I add them to the main block. As soon as I add d1 and d2 to the "main" block and call either of the two functions, both return two empty dictionaries. Why does this happen? How can I fix it? Please help!
Really just a series of somewhat careless beginner mistakes you can fix this way ...
python Syntax (Toggle Plain Text) 
  1. """assume file "filename.txt" has this content
  2. a,1
  3. b,2
  4. c,3
  5. d,4
  6. """
  7.  
  8. def junk(f):
  9.     d1 = {}
  10.     d2 = {}
  11.     for line in f:
  12.         columns = line.split(",")  # <---- delimiter is comma
  13.         letters = columns[0]       # <---- index starts with 0
  14.         numbers = columns[1]   # <---- index again
  15.         d1[letters] = numbers
  16.         d2[numbers] = letters
  17.     return d1, d2
  18.  
  19. def something(d1, d2):    # <---- give function proper arguments
  20.     print d1
  21.     print d2
  22.  
  23. if __name__ == "__main__":
  24.     f = open("filename.txt")
  25.     d1, d2 = junk(f)       # expand the tuple that is returned
  26.     something(d1, d2)  # give function its needed arguments
  27.  
  28. """my output -->
  29. {'a': '1\n', 'c': '3\n', 'b': '2\n', 'd': '4'}
  30. {'3\n': 'c', '1\n': 'a', '4': 'd', '2\n': 'b'}
  31. """
May 'the Google' be with you!
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  #7
Nov 8th, 2009
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Originally Posted by vegaseat View Post
Really just a series of somewhat careless beginner mistakes you can fix this way ...
python Syntax (Toggle Plain Text) 
  1. """assume file "filename.txt" has this content
  2. a,1
  3. b,2
  4. c,3
  5. d,4
  6. """
  7.  
  8. def junk(f):
  9.     d1 = {}
  10.     d2 = {}
  11.     for line in f:
  12.         columns = line.split(",")  # <---- delimiter is comma
  13.         letters = columns[0]       # <---- index starts with 0
  14.         numbers = columns[1]   # <---- index again
  15.         d1[letters] = numbers
  16.         d2[numbers] = letters
  17.     return d1, d2
  18.  
  19. def something(d1, d2):    # <---- give function proper arguments
  20.     print d1
  21.     print d2
  22.  
  23. if __name__ == "__main__":
  24.     f = open("filename.txt")
  25.     d1, d2 = junk(f)       # expand the tuple that is returned
  26.     something(d1, d2)  # give function its needed arguments
  27.  
  28. """my output -->
  29. {'a': '1\n', 'c': '3\n', 'b': '2\n', 'd': '4'}
  30. {'3\n': 'c', '1\n': 'a', '4': 'd', '2\n': 'b'}
  31. """
But how come if you call the function junk again after the program finished executing and showed the output, it will return two empty dicts?
Last edited by pyprog; Nov 8th, 2009 at 3:28 pm.
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  #8
Nov 8th, 2009
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Originally Posted by pyprog View Post
But how come if you call the function junk again after the program finished executing and showed the output, it will return two empty dicts?
Because the code is also poorly designed, ideally you should send the file name to the function, open the file there, read and close it.

You can fix your code this way ...
python Syntax (Toggle Plain Text) 
  1. def junk(fh):
  2.     d1 = {}
  3.     d2 = {}
  4.     for line in fh:
  5.         columns = line.split(",")
  6.         letters = columns[0]
  7.         numbers = columns[1]
  8.         d1[letters] = numbers
  9.         d2[numbers] = letters
  10.     return d1, d2
  11.  
  12. def something(d1, d2):
  13.     print d1
  14.     print d2
  15.  
  16. if __name__ == "__main__":
  17.     fh = open("filename.txt")
  18.     d1, d2 = junk(fh)
  19.     something(d1, d2)
  20.     print( '-'*50 )
  21.     # second time around
  22.     # file handle fh is at the end of the file now
  23.     # reset it to the start of the file
  24.     fh.seek(0)
  25.     d1, d2 = junk(fh)
  26.     something(d1, d2)
  27.  
  28. """my output -->
  29. {'a': '1\n', 'c': '3\n', 'b': '2\n', 'd': '4'}
  30. {'3\n': 'c', '1\n': 'a', '4': 'd', '2\n': 'b'}
  31. --------------------------------------------------
  32. {'a': '1\n', 'c': '3\n', 'b': '2\n', 'd': '4'}
  33. {'3\n': 'c', '1\n': 'a', '4': 'd', '2\n': 'b'}
  34. """
This would have been the better design ...
python Syntax (Toggle Plain Text) 
  1. """assume file "filename.txt" has this content
  2. a,1
  3. b,2
  4. c,3
  5. d,4
  6. """
  7.  
  8. def file2dictionary(fname):
  9.     """
  10.     read data from file and create a dictionary d1 and its reverse d2
  11.     """
  12.     fh = open("filename.txt")
  13.     d1 = {}
  14.     d2 = {}
  15.     for line in fh:
  16.         columns = line.split(",")
  17.         letters = columns[0]
  18.         numbers = columns[1]
  19.         d1[letters] = numbers
  20.         d2[numbers] = letters
  21.     fh.close()
  22.     return d1, d2
  23.  
  24. def print_dict(d1, d2):
  25.     """print the two dictionaries d1 and d2"""
  26.     print(d1)
  27.     print(d2)
  28.  
  29. if __name__ == "__main__":
  30.     fname = "filename.txt"
  31.     d1, d2 = file2dictionary(fname)
  32.     print_dict(d1, d2)
  33.     print( '-'*50 )
  34.     # test second time around
  35.     d1, d2 = file2dictionary(fname)
  36.     print_dict(d1, d2)
  37.  
  38. """my output -->
  39. {'a': '1\n', 'c': '3\n', 'b': '2\n', 'd': '4'}
  40. {'3\n': 'c', '1\n': 'a', '4': 'd', '2\n': 'b'}
  41. --------------------------------------------------
  42. {'a': '1\n', 'c': '3\n', 'b': '2\n', 'd': '4'}
  43. {'3\n': 'c', '1\n': 'a', '4': 'd', '2\n': 'b'}
  44. """
Note:
If you use the print() function instead of the print statement this code will work with Python2 and Python3.
Last edited by vegaseat; Nov 8th, 2009 at 4:02 pm.
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