Is Python a

Generating Both Offsets and Items: enumerateEarlier, we discussed using range to generate the offsets of items in a string, ratherthan the items at those offsets. In some programs, though, we need both: the item touse, plus an offset as we go. Traditionally, this was coded with a simple for loop thatalso kept a counter of the current offset:>>> S = 'spam'>>> offset = 0>>> for item in S:... print item, 'appears at offset', offset... offset += 1...s appears at offset 0p appears at offset 1a appears at offset 2m appears at offset 3This works, but in more recent Python releases, a new built-in named enumerate doesthe job for us:>>> S = 'spam'>>> for (offset, item) in enumerate(S):... print item, 'appears at offset', offset...s appears at offset 0p appears at offset 1a appears at offset 2m appears at offset 3The enumerate function returns a generator object—a kind of object that supports theiteration protocol we met earlier in this chapter, and will discuss in more detail in thenext part of the book. It has a next method that returns an (index, value) tuple eachtime through the list, which we can unpack with tuple assignment in the for (muchlike using zip):>>> E = enumerate(S)>>> E.next( )(0, 's')>>> E.next( )(1, 'p')As usual, we don’t normally see this machinery because iteration contexts—includinglist comprehensions, the subject of the next section—run the iteration protocolautomatically:>>> [c * i for (i, c) in enumerate(S)]['', 'p', 'aa', 'mmm']Loop Coding Techniques | 271