python 3.0 open() default encoding

I am trying to count the lines in a JSON file. Click HERE to access my JSON file .

I tried to use the below code to count the lines.

input = open("json/world_bank.json")
i=0
for l in input:
    i+=1
print(i)

But the above code is throwing a UniCodeDecode Error as shown below.

---------------------------------------------------------------------------
UnicodeDecodeError                        Traceback (most recent call last)
<ipython-input-17-edc88ade7225> in <module>()
      2 
      3 i=0
----> 4 for l in input:
      5     i+=1
      6 

C:\Users\Subbi Reddy\AppData\Local\Continuum\Anaconda3\lib\encodings\cp1252.py in decode(self, input, final)
     21 class IncrementalDecoder(codecs.IncrementalDecoder):
     22     def decode(self, input, final=False):
---> 23         return codecs.charmap_decode(input,self.errors,decoding_table)[0]
     24 
     25 class StreamWriter(Codec,codecs.StreamWriter):

UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 3979: character maps to <undefined>

Then i included encoding parameter in open function as shown below.

input = open("json/world_bank.json",encoding="utf8")

Then it started working and giving output as 500.

As far as i know python open should consider "utf8" as default encoding.

Where i am going wrong in here.

The default UTF-8 encoding of Python 3 only extends to byte->str conversions. open() instead uses your environment to choose an appropriate encoding:

From the Python 3 docs for open():

encoding is the name of the encoding used to decode or encode the file. This should only be used in text mode. The default encoding is platform dependent (whatever locale.getpreferredencoding() returns), but any text encoding supported by Python can be used. See the codecs module for the list of supported encodings.

In your case, as you're on Windows with a Western Europe/North America, you will be given the 8bit Windows-1252 character set. Setting encoding to utf-8 overrides this.