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Python Sequences

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Concatenation and Multiplication

3:04 with Ashley Boucher

Learn how to combine sequences with concatenation and multiplication

The python docs describe the effects of concatenating and multiplying immutable objects:

Concatenating immutable sequences always results in a new object. This means that building up a sequence by repeated concatenation will have a quadratic runtime cost in the total sequence length. To get a linear runtime cost, you must switch to one of the alternatives below:

  • if concatenating str objects, you can build a list and use str.join() at the end or else write to an io.StringIO instance and retrieve its value when complete

  • if concatenating bytes objects, you can similarly use bytes.join() or io.BytesIO, or you can do in-place concatenation with a bytearray object. bytearray objects are mutable and have an efficient overallocation mechanism

  • if concatenating tuple objects, extend a list instead

This might all sound rather complicated, but the gist is that sometimes new Python sequences are built up by looping over an existing sequence and concatenating each item onto the new sequence. Using a tuple or string for this would be very costly in terms of memory and time!

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    Welcome to the final lesson of Python sequences. 0:00
    In this video, we're going to be discussing concatenation and 0:03
    multiplication of Python sequences. 0:05
    Concatenation is when you attach one object to the end of another, so together, 0:08
    they become one larger object. 0:12
    For mutable sequence types, like lists, this concatenation can be done in place, 0:14
    the original list can become a larger list. 0:18
    For immutable sequence types, like tuples and 0:21
    strings, concatenation always involves creating an entirely new object. 0:23
    This is not very memory efficient. 0:27
    So if you're making repeated concatenations on an immutable type, 0:29
    you might want to consider a different data structure. 0:33
    Concatenation is done by using what looks like simple addition, 0:36
    the plus sign, let's take a look. 0:39
    Here I have two lists, object1 and object2. 0:41
    To concatenate, I just add them together. 0:44
    What do you think will happen if print out object1? 0:55
    Let's find out. 0:58
    As you can see, object1 now contains all the elements of object1 and object2. 1:12
    It's important to note that it doesn't matter if the sequences you're 1:18
    concatenating have any overlapping elements. 1:20
    Concatenation simply attaches the second object to the end of the first and 1:23
    ignores the contents. 1:27
    It won't try to merge the objects, remove duplicates, or 1:29
    change the order of a sequence in any way. 1:31
    Pause the video here and 1:34
    take a moment to play with concatenation in your attached workspace. 1:35
    What happens if you change the provided list to tuples? 1:38
    What if you change them to strings? 1:41
    When you're done exploring concatenation, come back, and 1:43
    we'll wrap up with multiplication. 1:46
    How did it go, did you learn anything new? 1:49
    Now, we'll move on to multiplication. 1:52
    This is basically concatenation multiple times. 1:54
    It's also called repetition. 1:57
    When you multiply a sequence, 1:59
    you concatenate that sequence to itself the provided number of times. 2:01
    The syntax for that is the sequence name, an asterisk, which means multiply, 2:05
    and then the number of times it should be repetitively concatenated. 2:10
    For this example, we'll use a short string. 2:14
    To multiply this string five times, I'll use the code, str*5. 2:22
    Now, let's wrap this in a print statement, And save it, and run it. 2:28
    All right, it printed out python concatenated together five times, 2:40
    great work. 2:45
    Check out the teacher's notes to learn more about immutability and 2:46
    concatenation and multiplication. 2:49
    See the following text step for 2:51
    more examples of both using different sequence types. 2:52
    I'm really proud of the work you've done in this course. 2:56
    Finish up with a code challenge and a quiz and celebrate your accomplishment, and 2:58
    don't forget, keep on coding. 3:02

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