Examples

Let’s type a few for loops interactively now, so you can see how they are used in practice.

Basic usage

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As mentioned earlier, a for loop can step across any kind of sequence object. In our first example, for instance, we’ll assign the name x to each of the three items in a list in turn, from left to right, and the print statement will be executed for each. Inside the print statement (the loop body), the name x refers to the current item in the list:

>>> for x in ["spam", "eggs", "ham"]:
...     print(x, end=' ')
...
spam eggs ham

The next two examples compute the sum and product of all the items in a list. Later in this chapter and later in the book we’ll meet tools that apply operations such as + and * to items in a list automatically, but it’s usually just as easy to use a for:

>>> sum = 0
>>> for x in [1, 2, 3, 4]:
...     sum = sum + x
...
>>> sum
10
>>> prod = 1
>>> for item in [1, 2, 3, 4]: prod *= item
...
>>> prod
24

Other data types

Any sequence works in a for, as it’s a generic tool. For example, for loops work on strings and tuples:

>>> S = "lumberjack"
>>> T = ("and", "I'm", "okay")

>>> for x in S: print(x, end=' ')     # Iterate over a string
...
l u m b e r j a c k

>>> for x in T: print(x, end=' ')     # Iterate over a tuple
...
and I'm okay

In fact, as we’ll in the next chapter when we explore the notion of “iterables,” for loops can even work on some objects that are not sequences—files and dictionaries work, too!

Tuple assignment in for loops

If you’re iterating through a sequence of tuples, the loop target itself can actually be a tuple of targets. This is just another case of the tuple-unpacking assignment we studied in Chapter 11 at work. Remember, the for loop assigns items in the sequence object to the target, and assignment works the same everywhere:

>>> T = [(1, 2), (3, 4), (5, 6)]
>>> for (a, b) in T:                   # Tuple assignment at work
...     print(a, b)
...
1 2
3 4
5 6

Here, the first time through the loop is like writing (a,b) = (1,2), the second time is like writing (a,b) = (3,4), and so on. The net effect is to automatically unpack the current tuple on each iteration.

This form is commonly used in conjunction with the zip call we’ll meet later in this chapter to implement parallel traversals. It also makes regular appearances in conjunction with SQL databases in Python, where query result tables are returned as sequences of sequences like the list used here—the outer list is the database table, the nested tuples are the rows within the table, and tuple assignment extracts columns.

Tuples in for loops also come in handy to iterate through both keys and values in dictionaries using the items method, rather than looping through the keys and indexing to fetch the values manually:

>>> D = {'a': 1, 'b': 2, 'c': 3}
>>> for key in D:
...    print(key, '=>', D[key])             # Use dict keys iterator and index
...
a => 1
c => 3
b => 2

>>> list(D.items())
[('a', 1), ('c', 3), ('b', 2)]

>>> for (key, value) in D.items():
...    print(key, '=>', value)              # Iterate over both keys and values
...
a => 1
c => 3
b => 2

It’s important to note that tuple assignment in for loops isn’t a special case; any assignment target works syntactically after the word for. Although we can always assign manually within the loop to unpack:

>>> T
[(1, 2), (3, 4), (5, 6)]

>>> for both in T:
...     a, b = both                         # Manual assignment equivalent
...     print(a, b)
...
1 2
3 4
5 6

Tuples in the loop header save us an extra step when iterating through sequences of sequences. As suggested in Chapter 11, even nested structures may be automatically unpacked this way in a for:

>>> ((a, b), c) = ((1, 2), 3)               # Nested sequences work too
>>> a, b, c
(1, 2, 3)

>>> for ((a, b), c) in [((1, 2), 3), ((4, 5), 6)]: print(a, b, c)
...
1 2 3
4 5 6

But this is no special case—the for loop simply runs the sort of assignment we ran just before it, on each iteration. Any nested sequence structure may be unpacked this way, just because sequence assignment is so generic:

>>> for ((a, b), c) in [([1, 2], 3), ['XY', 6]]: print(a, b, c)
...
1 2 3
X Y 6

Python 3.0 extended sequence assignment in for loops

In fact, because the loop variable in a for loop can really be any assignment target, we can also use Python 3.0’s extended sequence-unpacking assignment syntax here to extract items and sections of sequences within sequences. Really, this isn’t a special case either, but simply a new assignment form in 3.0 (as discussed in Chapter 11); because it works in assignment statements, it automatically works in for loops.

Consider the tuple assignment form introduced in the prior section. A tuple of values is assigned to a tuple of names on each iteration, exactly like a simple assignment statement:

>>> a, b, c = (1, 2, 3)                               # Tuple assignment
>>> a, b, c
(1, 2, 3)

>>> for (a, b, c) in [(1, 2, 3), (4, 5, 6)]:          # Used in for loop
...     print(a, b, c)
...
1 2 3
4 5 6

In Python 3.0, because a sequence can be assigned to a more general set of names with a starred name to collect multiple items, we can use the same syntax to extract parts of nested sequences in the for loop:

>>> a, *b, c = (1, 2, 3, 4)                           # Extended seq assignment
>>> a, b, c
(1, [2, 3], 4)

>>> for (a, *b, c) in [(1, 2, 3, 4), (5, 6, 7, 8)]:
...     print(a, b, c)
...
1 [2, 3] 4
5 [6, 7] 8

In practice, this approach might be used to pick out multiple columns from rows of data represented as nested sequences. In Python 2.X starred names aren’t allowed, but you can achieve similar effects by slicing. The only difference is that slicing returns a type-specific result, whereas starred names always are assigned lists:

>>> for all in [(1, 2, 3, 4), (5, 6, 7, 8)]:          # Manual slicing in 2.6
...     a, b, c = all[0], all[1:3], all[3]
...     print(a, b, c)
...
1 (2, 3) 4
5 (6, 7) 8

See Chapter 11 for more on this assignment form.

Nested for loops

Now let’s look at a for loop that’s a bit more sophisticated than those we’ve seen so far. The next example illustrates statement nesting and the loop else clause in a for. Given a list of objects (items) and a list of keys (tests), this code searches for each key in the objects list and reports on the search’s outcome:

>>> items = ["aaa", 111, (4, 5), 2.01]   # A set of objects
>>> tests = [(4, 5), 3.14]               # Keys to search for
>>>
>>> for key in tests:                    # For all keys
...     for item in items:               # For all items
...         if item == key:              # Check for match
...             print(key, "was found")
...             break
...     else:
...         print(key, "not found!")
...
(4, 5) was found
3.14 not found!

Because the nested if runs a break when a match is found, the loop else clause can assume that if it is reached, the search has failed. Notice the nesting here. When this code runs, there are two loops going at the same time: the outer loop scans the keys list, and the inner loop scans the items list for each key. The nesting of the loop else clause is critical; it’s indented to the same level as the header line of the inner for loop, so it’s associated with the inner loop, not the if or the outer for.

Note that this example is easier to code if we employ the in operator to test membership. Because in implicitly scans an object looking for a match (at least logically), it replaces the inner loop:

>>> for key in tests:                    # For all keys
...     if key in items:                 # Let Python check for a match
...         print(key, "was found")
...     else:
...         print(key, "not found!")
...
(4, 5) was found
3.14 not found!

In general, it’s a good idea to let Python do as much of the work as possible (as in this solution) for the sake of brevity and performance.

The next example performs a typical data-structure task with a for—collecting common items in two sequences (strings). It’s roughly a simple set intersection routine; after the loop runs, res refers to a list that contains all the items found in seq1 and seq2:

>>> seq1 = "spam"
>>> seq2 = "scam"
>>>
>>> res = []                             # Start empty
>>> for x in seq1:                       # Scan first sequence
...     if x in seq2:                    # Common item?
...         res.append(x)                # Add to result end
...
>>> res
['s', 'a', 'm']

Unfortunately, this code is equipped to work only on two specific variables: seq1 and seq2. It would be nice if this loop could somehow be generalized into a tool you could use more than once. As you’ll see, that simple idea leads us to functions, the topic of the next part of the book.

Why You Will Care: File Scanners

In general, loops come in handy anywhere you need to repeat an operation or process something more than once. Because files contain multiple characters and lines, they are one of the more typical use cases for loops. To load a file’s contents into a string all at once, you simply call the file object’s read method:

file = open('test.txt', 'r')   # Read contents into a string
print(file.read())

But to load a file in smaller pieces, it’s common to code either a while loop with breaks on end-of-file, or a for loop. To read by characters, either of the following codings will suffice:

file = open('test.txt')
while True:
    char = file.read(1)         # Read by character
    if not char: break
    print(char)

for char in open('test.txt').read():
    print(char)

The for loop here also processes each character, but it loads the file into memory all at once (and assumes it fits!). To read by lines or blocks instead, you can use while loop code like this:

file = open('test.txt')
while True:
    line = file.readline()      # Read line by line
    if not line: break
    print(line, end='')         # Line already has a \n

file = open('test.txt', 'rb')
while True:
    chunk = file.read(10)       # Read byte chunks: up to 10 bytes
    if not chunk: break
    print(chunk)

You typically read binary data in blocks. To read text files line by line, though, the for loop tends to be easiest to code and the quickest to run:

for line in open('test.txt').readlines():
    print(line, end='')

for line in open('test.txt'):   # Use iterators: best text input mode
    print(line, end='')

The file readlines method loads a file all at once into a line-string list, and the last example here relies on file iterators to automatically read one line on each loop iteration (iterators are covered in detail in Chapter 14). See the library manual for more on the calls used here. The last example here is generally the best option for text files—besides its simplicity, it works for arbitrarily large files and doesn’t load the entire file into memory all at once. The iterator version may be the quickest, but I/O performance is less clear-cut in Python 3.0.

In some 2.X Python code, you may also see the name open replaced with file and the file object’s older xreadlines method used to achieve the same effect as the file’s automatic line iterator (it’s like readlines but doesn’t load the file into memory all at once). Both file and xreadlines are removed in Python 3.0, because they are redundant; you shouldn’t use them in 2.6 either, but they may pop up in older code and resources. Watch for more on reading files in Chapter 36; as we’ll see there, text and binary files have slightly different semantics in 3.0.