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I am reading through Lutz's Programming Python and the following script is designed to create three records of people, make them persistent by storing them in a shelve, and then allow a user to enter people names in the terminal to get information back. I don't have all the code here, just what I think is necessary to understand.

Lines 8-13 show the class used in creating each person. These are then written to a shelf called class-shelve shown on line 15. The two lines I'm having trouble understanding are 17 and 29/30.

17 I am not sure of the logic here.

29 is the line from the book, and I don't understand how the code i try on 30 is any different?

  6 import shelve 
  7     
  8 class Person:
  9     def __init__(self, name, age, pay=0, job=None):
 10             self.name = name
 11             self.age = age
 12             self.pay = pay 
 13             self.job = job
 14 
 15 classShelve = './output/class-shelve'
 16 fieldnames = ('name', 'age', 'job', 'pay')
 17 maxfield = max(len(f) for f in fieldnames)
 18 db = shelve.open(classShelve)
 19 
 20 while True:
 21     key = input('\nKey? => ')
 22     if not key: break
 23     try:
 24         record = db[key]
 25     except:
 26         print('No such key "%s"!' % key)
 27     else:
 28         for field in fieldnames:
 29             #print(field.ljust(maxfield), '=>', getattr(record, field))
 30             print(field, '=>', getattr(record, field))
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    17 gives you the maximum length of the longest fieldname. It is decipherable almost in plain English. max(len(f) for f in fieldnames) => the max len(gth) of f for (all of the) f in fieldnames. For 29 v 30, there isn't really a difference, except the use of ljust. Is that your question? Commented Nov 20, 2015 at 21:39
  • That makes sense thanks. And yeah i have a basic understanding of what ljust is doing, but both lines 29/30 give me the same output and doesn't change justification. Commented Nov 20, 2015 at 21:45
  • It just makes the table cleaner. ljust and rjust don't do anything if the length of the string is already >= the argument they're given. Compare 'hello'.ljust(5) and 'hello'.ljust(20) (note all the trailing space after the latter, or use rjust if it's easier to see). Commented Nov 20, 2015 at 21:47
  • Oh cool that makes sense now. I appreciate the help. Commented Nov 20, 2015 at 21:49

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Line 17:

maxfield = max(len(f) for f in fieldnames)

this applies the max function over the list created by the comprehension len(f) for f in fieldnames

len(f) for f in fieldnames

could also be written as

[len(f) for f in fieldnames]

which means: create a list of the lengths of all the items in fieldnames.

max(len(f) for f in fieldnames)

means: find the biggest item in fieldnames.

Line 29/30

Your question is not so clear. I'm assuming that you are asking what the following function does?

field.ljust(maxfield)

According to this reference, string.ljust pads out a string to the given length so as to left justify it. By default, it pads with spaces. Perhaps that's why you're not seeing anything. If you supply it with a second argument, it uses that to pad the string. If the length of the string is greater than the supplied length, it just returns the string without modifying it.

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1 Comment

Slightly misleading breakdown of max(len(f) for f in fieldnames) because the comprehension doesn't return a list there. Without the [] inside a function call that becomes an implied generator expression, returning values one at a time. Otherwise, great answer.

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