given a json of a block, how do I calculate its hash? Here is the genesis block with its hash. How the hash was calculated?

{
  "hash":"000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f",
  "ver":1,
  "prev_block":"0000000000000000000000000000000000000000000000000000000000000000",
  "mrkl_root":"4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b",
  "time":1231006505,
  "bits":486604799,
  "nonce":2083236893,
  "n_tx":1,
  "size":285,
  "tx":[
    {
      "hash":"4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b",
      "ver":1,
      "vin_sz":1,
      "vout_sz":1,
      "lock_time":0,
      "size":204,
      "in":[
        {
          "prev_out":{
            "hash":"0000000000000000000000000000000000000000000000000000000000000000",
            "n":4294967295
          },
          "coinbase":"04ffff001d0104455468652054696d65732030332f4a616e2f32303039204368616e63656c6c6f72206f6e206272696e6b206f66207365636f6e64206261696c6f757420666f722062616e6b73"
        }
      ],
      "out":[
        {
          "value":"50.00000000",
          "scriptPubKey":"04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5f OP_CHECKSIG"
        }
      ]
    }
  ],
  "mrkl_tree":[
    "4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b"
  ]
}
As described on the wiki page about block hashing, the 2xSHA256 hash is taken of the block's header only (not the transactions in it): 80 bytes, consisting of the version, previous block hash, tx merkle root, time, bits and nonce.

Direct quote from that page :

&gt;&gt;&gt; import hashlib
&gt;&gt;&gt; header_hex = ("01000000" +
  "81cd02ab7e569e8bcd9317e2fe99f2de44d49ab2b8851ba4a308000000000000" +
  "e320b6c2fffc8d750423db8b1eb942ae710e951ed797f7affc8892b0f1fc122b" +
  "c7f5d74d" +
  "f2b9441a" +
   "42a14695")
&gt;&gt;&gt; header_bin = header_hex.decode('hex')
&gt;&gt;&gt; hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest()
&gt;&gt;&gt; hash.encode('hex_codec')
'1dbd981fe6985776b644b173a4d0385ddc1aa2a829688d1e0000000000000000'
&gt;&gt;&gt; hash[::-1].encode('hex_codec')
'00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d'