如何在 Python 中存储递归函数的输出？

Question

I have created a dictionary like so:

d = {1: {3: {}, 4: {6: {}}}, 5: {}}

Then I iterate through all the items and I can do it recursively:

def pretty(d, indent=0):
    for key, value in d.items():
        print('\t' * indent + str(key))

        if isinstance(value, dict):
            pretty(value, indent+1)
        else:
            print('\t' * (indent+1) + str(value))

pretty(d)

My goal is to store the output into a string variable in such a way that I can manipulate it. Therefore the result should be something like this:

msg ="""
1
        3
        4
                6
5
"""

I tried to reach my goal with the following implementation:

def pretty(d, indent=0, indent_before=0, msg_old=""):
    for key, value in d.items():
        #print('\t' * indent + str(key) + '({})({})'.format(indent,indent_before))
        msg = msg_old+'\t' * indent + str(key) + '({})({})\n'.format(indent,indent_before)
        if isinstance(value, dict):
            pretty(value, indent+1, indent, msg)
        else:
            print('\t' * (indent+1) + str(value))
    return msg
    
msg = pretty(result)
print(msg)

But the output of my attempt is: None

Would you be able to suggest a smart and elegant way to achieve the desired result?

Answer 1

The main problem is that in your recursive call you are discarding the return value (ie calling pretty but not appending the return value to the existing message).

Here is a solution based closely on your original code.

d = {1: {3: {}, 4: {6: {}}}, 5: {}}

def pretty(d, indent=0):
    msg = ''
    for key, value in d.items():
        msg += '\t' * indent + str(key) + '\n'
        if isinstance(value, dict):
            msg += pretty(value, indent+1)  # <=== see how return value is used
        else:
            msg += '\t' * (indent+1) + str(value) + '\n'
    return msg

print(pretty(d))

Answer 2

I know you are trying to implement your own solution but you never mentioned the builtin pprint library/function that does this all for you:

from pprint import pformat
d = {1: {3: {}, 4: {6: {}}}, 5: {}}
print(pformat(d, indent=0))

---

{1: {3: {}, 4: {6: {}}}, 5: {}}

Answer 3

If your dict is well-formed, then you can use:

>>> def pretty(d, indent=0):
...     return "".join(["\n" + "\t" * indent + str(k) +
...                     pretty(v, indent + 1) for k, v in d.items()])
...
>>> d = {1: {3: {}, 4: {6: {}}}, 5: {}}
>>> pretty(d)
'\n1\n\t3\n\t4\n\t\t6\n5'
>>> print(pretty(d))

1
        3
        4
                6
5