What should I know in Python?

This was posted on SO (and promptly closed!):

What questions should every good Python developer be able to answer?

Alex Martelli, author of the Python Cookbook and Python guru extraordinaire, gave these four questions:

• how do you sort a list of dicts by the value each has for key 'foo'?

There's a really nice page on sorting here. Recall that the built-in function sorted takes a keyword argument key. I usually define a function to feed to sorted,

>>> L = [{'foo':1}, {'foo':3}, {'foo':10}] >>> def f(e): return e['foo'] ... >>> sorted(L, key=f) [{'foo': 1}, {'foo': 3}, {'foo': 10}]

but the cool kids like lambdas:

>>> import operator >>> sorted(L, key = operator.itemgetter('foo')) [{'foo': 1}, {'foo': 3}, {'foo': 10}]

sorted also can do reverse:

>>> sorted(L, key=f, reverse=True) [{'foo': 10}, {'foo': 3}, {'foo': 1}]

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• how do you get the highest 10 integers out of a list of a million integers?

The built-in function sort can do this in an instant.

>>> import random >>> N = int(1E6) >>> L = [random.choice(xrange(N)) for i in range(N)] >>> L.sort() >>> L[-5:] [999990, 999991, 999992, 999994, 999998]

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• how do you sort a list of strings in case-insensitive alphabetical order?

Feed the built-in function str.lower to sort

>>> FH = open('/usr/share/dict/words') >>> words = FH.read().strip().split('\n') >>> FH.close() >>> len(words) 234936 >>> L = [random.choice(words) for i in range(1000)] >>> L.sort(key=str.lower) >>> for w in L[10:15]: ... print w ... adenophthalmia advenient Aegisthus afterpast afterripening

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• given a list of ints, how do you make a string with their str forms separated by spaces?

This one's trivial.

>>> L = range(10) >>> print ''.join([str(n) for n in L]) 0123456789