How beautiful print Python objects? The standard library can be easily achieved

Not long ago, I wrote a review article in Python print history of the development , referred to the development of two clues:

Open wire: Early print statement with shadow C and Shell, is a application-level statement, in the first ten years, we have experienced PEP-214 and PEP-259 improvements; and then in 2009 the big version 3.0, by the statement into a print () function, is still version 3.3, enhancements once did, eventually rising to become first-class built-in functions.
Dark line: The print competitors, like a traditional log module logging, debugging module pdb, mainstream IDE debugging functions, as well as a rising star PySnooper, they aimed at the location of print, gearing up, like a tiger.

This article is still relevant with print, I would like to introduce the standard library pprintmodules.

(Without authorization, please do not reprint the article starting on:. Https://mp.weixin.qq.com/s/d1dQH1Fqaj_U3AXKYluhSw )

pprint is "pretty printer" shorthand, "pretty" means "beautiful, beautiful", and indicates the degree of tone "fairly", so its meaning is quite :() beautiful print.

This is a fairly simple but with modules, mainly for printing objects complex data structures, such as nested list of tuples, and dictionaries.

Take a look at print () for printing an example:

mylist = ["Beautiful is better than ugly.", "Explicit is better than implicit.", "Simple is better than complex.", "Complex is better than complicated."]

print(mylist)

# 结果如下：
['Beautiful is better than ugly.', 'Explicit is better than implicit.', 'Simple is better than complex.', 'Complex is better than complicated.']

This is a simple example, all printed on one line.

Imagine if the object element is nested content (such as complex Json data), or have a huge number of elements (for example, save a lot URL link in the list), then print out will be like?

It certainly is a mess of bad reading.

Use pprint module pprint () alternate print (), pain points can be resolved as follows:

Suitable provided width of the line, make appropriate linefeed
Set the indent printing hierarchy format printing
判断对象中是否出现无限循环，并优化打印内容

1、简单使用

语法：pprint(object, stream=None, indent=1, width=80, depth=None, *,compact=False)

默认的行宽度参数为 80，当打印的字符（character）小于 80 时，pprint() 基本上等同于内置函数 print()，当字符超出时，它会作美化，进行格式化输出：

import pprint

# 打印上例的 mylist
pprint.pprint(mylist)

# 打印的元素是换行的（因为超出80字符）：
['Beautiful is better than ugly.',
 'Explicit is better than implicit.',
 'Simple is better than complex.',
 'Complex is better than complicated.']

2、设置缩进为 4 个空格（默认为1）

pprint.pprint(mylist, indent=4)

[   'Beautiful is better than ugly.',
    'Explicit is better than implicit.',
    'Simple is better than complex.',
    'Complex is better than complicated.']

3、设置打印的行宽

mydict = {'students': [{'name':'Tom', 'age': 18},{'name':'Jerry', 'age': 19}]}

pprint.pprint(mydict)

# 未超长：
{'students': [{'age': 18, 'name': 'Tom'}, {'age': 19, 'name': 'Jerry'}]}

pprint.pprint(mydict, width=20)

# 超长1：
{'students': [{'age': 18,
               'name': 'Tom'},
              {'age': 19,
               'name': 'Jerry'}]}

pprint.pprint(mydict, width=70)

# 超长2：
{'students': [{'age': 18, 'name': 'Tom'},
              {'age': 19, 'name': 'Jerry'}]}

4、设置打印的层级（默认全打印）

newlist = [1, [2, [3, [4, [5]]]]]

pprint.pprint(newlist, depth=3)

# 超出的层级会用...表示
[1, [2, [3, [...]]]]

5、优化循环结构的打印

当列表或其它数据结构中出现循环引用时，要完整打印出所有内容是不可能的。

所以 print 作了简化处理，就像上例一样，只打印外层的壳，而不打印内层循环的东西。

这种处理方式是简化了，但没有指出是谁导致了循环，还容易看漏。

pprint() 方法作了改进，遇到无限循环结构时，会表示成<Recursion on typename with id=number> 的格式。

还有个 saferepr() 方法，也是这样优化，而且返回的是个字符串：

newlist = [1, 2]
newlist.insert(0, newlist)

# 列表元素指向列表自身，造成循环引用
# 直接 print 的结果是：[[...], 1, 2]

pprint.pprint(newlist)
# [<Recursion on list with id=1741283656456>, 1, 2]

pprint.saferepr(newlist)
# '[<Recursion on list with id=1741283656456>, 1, 2]'

6、判断是否出现循环结构

有两个方法可以判断一个对象中是否出现无限循环：

pprint.isrecursive(newlist)
# True

pprint.isreadable(newlist)
# False

isreadable() 除了能像 isrecursive() 一样判断循环，还能判断该格式化内容是否可被 eval() 重构。

以上就是 pprint 模块的快捷入门介绍，除此之外，还有 pformat() 方法、PrettyPrinter 类，以及某些参数的使用等内容，我觉得没有大用，就不多说了。

如若感兴趣，你可查阅：

官方介绍：https://docs.python.org/zh-cn/3/library/pprint.html
源码地址：https://github.com/python/cpython/blob/3.7/Lib/pprint.py

最后，还有两个小小的点：

1、用 pprint() 替换 print() 的技巧

在不考虑 print() 函数本身的参数的情况下，可以在引入 pprint 模块后，写上 “print = pprint.pprint”，令 print() 起到改头换面的效果：

import pprint
print = pprint.pprint

mylist = ["Beautiful is better than ugly.", "Explicit is better than implicit.", "Simple is better than complex.", "Complex is better than complicated."]

print(mylist)

# 可对比本文开头的例子
['Beautiful is better than ugly.',
 'Explicit is better than implicit.',
 'Simple is better than complex.',
 'Complex is better than complicated.']

2、国人开发的 beeprint

国内某位 pan 同学在 Github 开源了个beeprint，明显是对标 pprint 的。

项目地址：https://github.com/panyanyany/beeprint

它优化了字典对象的打印，对于从其它语言转过来的同学而言（例如 Java），这是个福音：

它还优化了长文本的打印，支持自定义对象的打印，看起来不错。

但是，其它功能不够齐全，而且作者停止维护两年了，荒废已久……

总体而言，pprint 算是 print() 的轻量级替代，简单实用，极其方便（毕竟是标准库），文档丰富而有保障。

所以，若想要打印美观易读的数据，这个 pprint 标准库，不妨一试哦。

作者简介：豌豆花下猫，生于广东毕业于武大，现为苏漂程序员，有一些极客思维，也有一些人文情怀，有一些温度，还有一些态度。公众号：「Python猫」（python_cat）

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How beautiful print Python objects? The standard library can be easily achieved

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