Pretty-print in Python: pprint
In Python, you can pretty-print objects such as lists (list) and dictionaries (dict) with the pprint module.
In the sample code, the list of dictionaries is used as an example. The pprint module is included in the standard library, so no additional installation is required, but you need to import it.
import pprint
l = [{'Name': 'Alice XXX', 'Age': 40, 'Points': [80, 20]},
{'Name': 'Bob YYY', 'Age': 20, 'Points': [90, 10]},
{'Name': 'Charlie ZZZ', 'Age': 30, 'Points': [70, 30]}]
See the following article for details about the print() function.
The textwrap module is useful for wrapping or truncating long strings, not lists or dictionaries.
Basic usage of pprint
The print() function outputs the elements of a list or dictionary on a single line without any line breaks.
print(l)
# [{'Name': 'Alice XXX', 'Age': 40, 'Points': [80, 20]}, {'Name': 'Bob YYY', 'Age': 20, 'Points': [90, 10]}, {'Name': 'Charlie ZZZ', 'Age': 30, 'Points': [70, 30]}]
With pprint.pprint(), each element of the list will be broken into a new line, as shown below, making it easier to read.
pprint.pprint(l)
# [{'Age': 40, 'Name': 'Alice XXX', 'Points': [80, 20]},
# {'Age': 20, 'Name': 'Bob YYY', 'Points': [90, 10]},
# {'Age': 30, 'Name': 'Charlie ZZZ', 'Points': [70, 30]}]
The line break position is determined by the argument settings described below.
Note that, as shown in the example above, the elements of the dictionary are sorted in the order of the keys. If the parameter sort_dicts, added in Python 3.8, is set to False (default: True), the order of the dictionaries is kept, but in earlier versions, they are always sorted.
If you want to convert the pretty-printed output to a string (str) instead of directly printing it, use pprint.pformat() described below.
Specify the output width (number of characters): width
The output width (number of characters) can be specified with width.
The line breaks to fit the specified number of characters. The default value is width=80.
pprint.pprint(l, width=40)
# [{'Age': 40,
# 'Name': 'Alice XXX',
# 'Points': [80, 20]},
# {'Age': 20,
# 'Name': 'Bob YYY',
# 'Points': [90, 10]},
# {'Age': 30,
# 'Name': 'Charlie ZZZ',
# 'Points': [70, 30]}]
If width is large, no newline is inserted, and the output is the same as print().
pprint.pprint(l, width=200)
# [{'Age': 40, 'Name': 'Alice XXX', 'Points': [80, 20]}, {'Age': 20, 'Name': 'Bob YYY', 'Points': [90, 10]}, {'Age': 30, 'Name': 'Charlie ZZZ', 'Points': [70, 30]}]
A line breaks at an element of a list or a dictionary, not between the key and value of a dictionary, nor in the middle of a number. Therefore, it does not always fit into the width of the number of characters specified by width.
Note that strings may be broken into a new line for each word.
pprint.pprint(l, width=1)
# [{'Age': 40,
# 'Name': 'Alice '
# 'XXX',
# 'Points': [80,
# 20]},
# {'Age': 20,
# 'Name': 'Bob '
# 'YYY',
# 'Points': [90,
# 10]},
# {'Age': 30,
# 'Name': 'Charlie '
# 'ZZZ',
# 'Points': [70,
# 30]}]
Specify the output depth: depth
The output depth can be specified with depth. The depth here means the depth of nesting.
Elements nested deeper than the specified value are printed with the ellipsis ....
pprint.pprint(l, depth=1)
# [{...}, {...}, {...}]
pprint.pprint(l, depth=2)
# [{'Age': 40, 'Name': 'Alice XXX', 'Points': [...]},
# {'Age': 20, 'Name': 'Bob YYY', 'Points': [...]},
# {'Age': 30, 'Name': 'Charlie ZZZ', 'Points': [...]}]
The default value is depth=None, and all elements are output.
You can specify both width and depth at the same time. The depth specifies the depth of the data structure, not the number of lines. The line break positions depend on the number of characters specified by width.
pprint.pprint(l, depth=2, width=40)
# [{'Age': 40,
# 'Name': 'Alice XXX',
# 'Points': [...]},
# {'Age': 20,
# 'Name': 'Bob YYY',
# 'Points': [...]},
# {'Age': 30,
# 'Name': 'Charlie ZZZ',
# 'Points': [...]}]
Specify the indent width: indent
The indent width can be specified with indent. The default value is indent=1.
pprint.pprint(l, indent=4, width=4)
# [ { 'Age': 40,
# 'Name': 'Alice '
# 'XXX',
# 'Points': [ 80,
# 20]},
# { 'Age': 20,
# 'Name': 'Bob '
# 'YYY',
# 'Points': [ 90,
# 10]},
# { 'Age': 30,
# 'Name': 'Charlie '
# 'ZZZ',
# 'Points': [ 70,
# 30]}]
Reduce line breaks: compact
By default, all elements of a list or dictionary break lines if they do not fit into width.
l_long = [list(range(10)), list(range(100, 110))]
print(l_long)
# [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [100, 101, 102, 103, 104, 105, 106, 107, 108, 109]]
pprint.pprint(l_long, width=40)
# [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
# [100,
# 101,
# 102,
# 103,
# 104,
# 105,
# 106,
# 107,
# 108,
# 109]]
If compact is set to True, elements that can fit within width are printed on a single line. This setting is particularly useful for lists with many elements.
pprint.pprint(l_long, width=40, compact=True)
# [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
# [100, 101, 102, 103, 104, 105, 106,
# 107, 108, 109]]
Note that compact was added in Python 3.4, so it cannot be used in earlier versions.
Convert to string: pprint.pformat()
You can convert dictionaries and lists to strings using str(). It produces a one-line string without line breaks, similar to the output of print().
s_normal = str(l)
print(s_normal)
# [{'Name': 'Alice XXX', 'Age': 40, 'Points': [80, 20]}, {'Name': 'Bob YYY', 'Age': 20, 'Points': [90, 10]}, {'Name': 'Charlie ZZZ', 'Age': 30, 'Points': [70, 30]}]
print(type(s_normal))
# <class 'str'>
You can use pprint.pformat() to get the output of pprint.pprint() as the string str.
s_pp = pprint.pformat(l)
print(s_pp)
# [{'Age': 40, 'Name': 'Alice XXX', 'Points': [80, 20]},
# {'Age': 20, 'Name': 'Bob YYY', 'Points': [90, 10]},
# {'Age': 30, 'Name': 'Charlie ZZZ', 'Points': [70, 30]}]
print(type(s_pp))
# <class 'str'>
The parameters of pprint.pformat() is the same as pprint.pprint().
s_pp = pprint.pformat(l, depth=2, width=40, indent=2)
print(s_pp)
# [ { 'Age': 40,
# 'Name': 'Alice XXX',
# 'Points': [...]},
# { 'Age': 20,
# 'Name': 'Bob YYY',
# 'Points': [...]},
# { 'Age': 30,
# 'Name': 'Charlie ZZZ',
# 'Points': [...]}]
Example: Pretty-print a list of lists
A list of lists can be difficult to read when printed with print(). For better readability, consider using pprint.pprint().
l_2d = [list(range(10)), list(range(10)), list(range(10))]
print(l_2d)
# [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]]
pprint.pprint(l_2d)
# [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]]
As mentioned above, where to start a new line is determined by the number of characters specified by width.
If the number of elements is small, it fits into the default output width width=80, so there is no line break.
l_2d = [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
print(l_2d)
# [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
pprint.pprint(l_2d)
# [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
If an appropriate width is specified, the line will be broken.
pprint.pprint(l_2d, width=20)
# [[0, 1, 2],
# [3, 4, 5],
# [6, 7, 8]]
If you want to get it as a string, use pprint.pformat().
s = pprint.pformat(l_2d, width=20)
print(s)
# [[0, 1, 2],
# [3, 4, 5],
# [6, 7, 8]]
print(type(s))
# <class 'str'>