How to use pprint to print an object using the built-in __str__(self) method?

问题

I have a Python script which processes a .txt file which contains report usage information. I'd like to find a way to cleanly print the attributes of an object using pprint's pprint(vars(object)) function.

The script reads the file and creates instances of a Report class. Here's the class.

class Report(object):
    def __init__(self, line, headers):
        self.date_added=get_column_by_header(line,headers,"Date Added")
        self.user=get_column_by_header(line,headers,"Login ID")
        self.report=get_column_by_header(line,headers,"Search/Report Description")
        self.price=get_column_by_header(line,headers,"Price")
        self.retail_price=get_column_by_header(line,headers,"Retail Price")

    def __str__(self):
        from pprint import pprint
        return str(pprint(vars(self)))

I'd like to be able to print instances of Report cleanly a-la-pprint.

for i,line in enumerate(open(path+file_1,'r')):
    line=line.strip().split("|")
    if i==0:
        headers=line

    if i==1:
        record=Report(line,headers)
        print record

When I call

print record

for a single instance of Report, this is what I get in the shell.

{'date_added': '1/3/2012 14:06',
'price': '0',
'report': 'some_report',
'retail_price': '0.25',
'user': 'some_username'}
 None

My question is two-fold.

First, is this a good / desired way to print an object's attributes cleanly? Is there a better way to do this with or without pprint?

Second, why does

None

print to the shell at the end? I'm confused where that's coming from.

Thanks for any tips.

回答1:

pprint is just another form of print. When you say pprint(vars(self)) it prints vars into stdout and returns none because it is a void function. So when you cast it to a string it turns None (returned by pprint) into a string which is then printed from the initial print statement. I would suggest changing your print to pprint or redefine print as print if its all you use it for.

def __str__(self):
    from pprint import pprint
    return str(vars(self))

for i,line in enumerate(open(path+file_1,'r')):
    line = line.strip().split("|")
    if i == 0:
        headers = line
    if i == 1:
        record = Report(line,headers)
        pprint record

One alternative is to use a formatted output:

def __str__(self):
    return "date added:   %s\nPrice:        %s\nReport:       %s\nretail price: %s\nuser:         %s" % tuple([str(i) for i in vars(self).values()])

Hope this helped

回答2:

Dan's solution is just wrong, and Ismail's in incomplete.

__str__() is not called, __repr__() is called.
__repr__() should return a string, as pformat does.
print normally indents only 1 character and tries to save lines. If you are trying to figure out structure, set the width low and indent high.

Here is an example

class S:
    def __repr__(self):
        from pprint import pformat
        return pformat(vars(self), indent=4, width=1)

a = S()
a.b = 'bee'
a.c = {'cats': ['blacky', 'tiger'], 'dogs': ['rex', 'king'] }
a.d = S()
a.d.more_c = a.c

print(a)

This prints

{   'b': 'bee',
    'c': {   'cats': [   'blacky',
                         'tiger'],
             'dogs': [   'rex',
                         'king']},
    'd': {   'more_c': {   'cats': [   'blacky',
                               'tiger'],
                  'dogs': [   'rex',
                              'king']}}}

Which is not perfect, but passable.

回答3:

pprint.pprint doesn't return a string; it actually does the printing (by default to stdout, but you can specify an output stream). So when you write print record, record.__str__() gets called, which calls pprint, which returns None. str(None) is 'None', and that gets printed, which is why you see None.

You should use pprint.pformat instead. (Alternatively, you can pass a StringIO instance to pprint.)

回答4:

For pretty-printing objects which contain other objects, etc. pprint is not enough. Try IPython's lib.pretty, which is based on a Ruby module.

from IPython.lib.pretty import pprint
pprint(complex_object)

回答5:

I think beeprint is what you need.

Just pip install beeprint and change your code to:

def __str__(self):
    from beeprint import pp
    return pp(self, output=False)

来源：https://stackoverflow.com/questions/9135485/how-to-use-pprint-to-print-an-object-using-the-built-in-str-self-method

标签

python

string

pprint