Splitting, Concatenating, and Joining Strings in Python

Splitting Without Parameters

Before going deeper, let’s look at a simple example:

>>> 'this is my string'.split()
['this', 'is', 'my', 'string']

This is actually a special case of a .split() call, which I chose for its simplicity. Without any separator specified, .split() will count any whitespace as a separator.

Another feature of the bare call to .split() is that it automatically cuts out leading and trailing whitespace, as well as consecutive whitespace. Compare calling .split() on the following string without a separator parameter and with having ' ' as the separator parameter:

>>> s = ' this   is  my string '
>>> s.split()
['this', 'is', 'my', 'string']
>>> s.split(' ')
['', 'this', '', '', 'is', '', 'my', 'string', '']

The first thing to notice is that this showcases the immutability of strings in Python: subsequent calls to .split() work on the original string, not on the list result of the first call to .split().

The second—and the main—thing you should see is that the bare .split() call extracts the words in the sentence and discards any whitespace.

Specifying Separators

.split(' '), on the other hand, is much more literal. When there are leading or trailing separators, you’ll get an empty string, which you can see in the first and last elements of the resulting list.

Where there are multiple consecutive separators (such as between “this” and “is” and between “is” and “my”), the first one will be used as the separator, and the subsequent ones will find their way into your result list as empty strings.

Limiting Splits With Maxsplit

.split() has another optional parameter called maxsplit. By default, .split() will make all possible splits when called. When you give a value to maxsplit, however, only the given number of splits will be made. Using our previous example string, we can see maxsplit in action:

>>> s = "this is my string"
>>> s.split(maxsplit=1)
['this', 'is my string']

As you see above, if you set maxsplit to 1, the first whitespace region is used as the separator, and the rest are ignored. Let’s do some exercises to test out everything we’ve learned so far.

Name,Phone,Address
Mike Smith,15554218841,123 Nice St, Roy, NM, USA
Anita Hernandez,15557789941,425 Sunny St, New York, NY, USA
Guido van Rossum,315558730,Science Park 123, 1098 XG Amsterdam, NL

[
    ['Mike Smith', '15554218841', '123 Nice St, Roy, NM, USA'],
    ['Anita Hernandez', '15557789941', '425 Sunny St, New York, NY, USA'],
    ['Guido van Rossum', '315558730', 'Science Park 123, 1098 XG Amsterdam, NL']
]

input_string = """Name,Phone,Address
Mike Smith,15554218841,123 Nice St, Roy, NM, USA
Anita Hernandez,15557789941,425 Sunny St, New York, NY, USA
Guido van Rossum,315558730,Science Park 123, 1098 XG Amsterdam, NL"""

def string_split_ex(unsplit):
    results = []

    # Bonus points for using splitlines() here instead, 
    # which will be more readable
    for line in unsplit.split('\n')[1:]:
        results.append(line.split(',', maxsplit=2))

    return results

print(string_split_ex(input_string))

Concatenating With the + Operator

There are a few ways of doing this, depending on what you’re trying to achieve. The simplest and most common method is to use the plus symbol (+) to add multiple strings together. Simply place a + between as many strings as you want to join together:

>>> 'a' + 'b' + 'c'
'abc'

In keeping with the math theme, you can also multiply a string to repeat it:

>>> 'do' * 2
'dodo'

Remember, strings are immutable! If you concatenate or repeat a string stored in a variable, you will have to assign the new string to another variable in order to keep it.

>>> orig_string = 'Hello'
>>> orig_string + ', world'
'Hello, world'
>>> orig_string
'Hello'
>>> full_sentence = orig_string + ', world'
>>> full_sentence
'Hello, world'

If we didn’t have immutable strings, full_sentence would instead output 'Hello, world, world'.

Another note is that Python does not do implicit string conversion. If you try to concatenate a string with a non-string type, Python will raise a TypeError:

>>> 'Hello' + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be str, not int

This is because you can only concatenate strings with other strings, which may be new behavior for you if you’re coming from a language like JavaScript, which attempts to do implicit type conversion.

Going From a List to a String in Python With .join()

There is another, more powerful, way to join strings together. You can go from a list to a string in Python with the join() method.

The common use case here is when you have an iterable—like a list—made up of strings, and you want to combine those strings into a single string. Like .split(), .join() is a string instance method. If all of your strings are in an iterable, which one do you call .join() on?

This is a bit of a trick question. Remember that when you use .split(), you call it on the string or character you want to split on. The opposite operation is .join(), so you call it on the string or character you want to use to join your iterable of strings together:

>>> strings = ['do', 're', 'mi']
>>> ','.join(strings)
'do,re,mi'

Here, we join each element of the strings list with a comma (,) and call .join() on it rather than the strings list.

>>> strings = ['do', 're', 'mi']
>>> ', '.join(strings)
'do, re, mi'

.join() is smart in that it inserts your “joiner” in between the strings in the iterable you want to join, rather than just adding your joiner at the end of every string in the iterable. This means that if you pass an iterable of size 1, you won’t see your joiner:

>>> 'b'.join(['a'])
'a'

[
    ['Boston', 'MA', '76F', '65% Precip', '0.15 in'],
    ['San Francisco', 'CA', '62F', '20% Precip', '0.00 in'],
    ['Washington', 'DC', '82F', '80% Precip', '0.19 in'],
    ['Miami', 'FL', '79F', '50% Precip', '0.70 in']
]

"""
Boston,MA,76F,65% Precip,0.15in
San Francisco,CA,62F,20% Precip,0.00 in
Washington,DC,82F,80% Precip,0.19 in
Miami,FL,79F,50% Precip,0.70 in
"""

input_list = [
    ['Boston', 'MA', '76F', '65% Precip', '0.15 in'],
    ['San Francisco', 'CA', '62F', '20% Precip', '0.00 in'],
    ['Washington', 'DC', '82F', '80% Precip', '0.19 in'],
    ['Miami', 'FL', '79F', '50% Precip', '0.70 in']
]

# We start with joining each inner list into a single string
joined = [','.join(row) for row in input_list]

# Now we transform the list of strings into a single string
output = '\n'.join(joined)

print(output)