[Python-ideas] Repr of lambda

[Steve Barnes]

On 18/12/2017 06:11, Ivan Pozdeev via Python-ideas wrote:
> On 17.12.2017 22:20, Serhiy Storchaka wrote:
>> Currently repr of doesn't contain much of information besides that it 
>> is a lambda.
>>
>> >>> lambda x: x**2
>> <function <lambda> at 0x7f3479b74488>
>>
>> All lambdas have the same repr, different only by unreadable 
>> hexadecimal address.
>>
>> What if include the signature and the expression of the lambda in its 
>> repr?
>>
>> >>> lambda x: x**2
>> <lambda x: x**2>
>>
> It's the same for named functions:
> 
>      In [1]: def ditto(a): return a
> 
>      In [2]: ditto
>      Out[2]: <function __main__.ditto>
> 
> Are you intending to do the same for them?
>> This would return an old feature of Python 0.9.1 
>> (https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Ftwitter.com%2Fdabeaz%2Fstatus%2F934835068043956224&data=02%7C01%7C%7C44a37122957d43015aa308d545de3321%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636491742990321352&sdata=iAE6MDdsZJDHfUqlHlPjnf2XV%2BiRZ%2BrP%2FL%2BIQ8kKoKo%3D&reserved=0). 
>>
>>
>> Repr of function could contain just the signature.
>>
>> <function foo(bar, baz)>
>>
>> But there is a problem with default values. Their reprs can be very 
>> long, especially in the following case with mutable default value:
>>
>> def foo(x, _cache={}):
>>     try:
>>         return _cache[x]
>>     except KeyError:
>>         pass
>>     _cache[x] = y = expensive_calculation(x)
>>     return y
>>
>> Maybe the placeholder should be always used instead of default values.
>>
>> _______________________________________________

Isn't this exactly the sort of information already available via 
inspect.getardspec, inspect.getsourcelines & inspect.getsource?

In [19]: import inspect

In [20]: l = lambda x: x**2

In [21]: inspect.getargspec(l)
Out[21]: ArgSpec(args=['x'], varargs=None, keywords=None, defaults=None)

In [22]: inspect.getsource(l)
Out[22]: u'l = lambda x: x**2\n'

In [23]:
     ...: def foo(x, _cache={}):
     ...:     try:
     ...:         return _cache[x]
     ...:     except KeyError:
     ...:         pass
     ...:     _cache[x] = y = expensive_calculation(x)
     ...:     return y
     ...:

In [24]: inspect.getargspec(foo)
Out[24]: ArgSpec(args=['x', '_cache'], varargs=None, keywords=None, 
defaults=({},))

In [25]: inspect.getsource(foo)
Out[25]: u'def foo(x, _cache={}):\n    try:\n        return _cache[x]\n 
   except KeyError:\n        pass\n    _cache[x] = y = 
expensive_calculation(x)\n    return y \n'

In [26]: inspect.getsourcelines(foo)
Out[26]:
([u'def foo(x, _cache={}):\n',
   u'    try:\n',
   u'        return _cache[x]\n',
   u'    except KeyError:\n',
   u'        pass\n',
   u'    _cache[x] = y = expensive_calculation(x)\n',
   u'    return y \n'],
  2)



-- 
Steve (Gadget) Barnes
Any opinions in this message are my personal opinions and do not reflect 
those of my employer.

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