How to calculate a packet checksum without sending it?

I've also tried to avoid show2() because it print the packet. I've found in the source a better solution:

del packet.chksum
packet = packet.__class__(bytes(packet))

This code regenerate the packet with the correct checksum without any print and actually is what show2() run in the background before printing.

You need to delete the .chksum value from the packet after you create it; then call .show2()

>>> from scapy.layers.inet import IP
>>> from scapy.layers.inet import ICMP
>>> from scapy.layers.inet import TCP
>>> target = "10.9.8.7"
>>> ttl = 64
>>> id = 32711
>>> sport = 2927
>>> dport = 80
>>> pak = IP(dst=target, src = "100.99.98.97", ttl=ttl, flags="DF", id=id, len=1200, chksum = 0)/TCP(flags="S", sport=sport, dport=int(dport), options=[('Timestamp',(0,0))], chksum = 0)
>>> del pak[IP].chksum
>>> del pak[TCP].chksum
>>> pak.show2()
###[ IP ]###
  version   = 4L
  ihl       = 5L
  tos       = 0x0
  len       = 1200
  id        = 32711
  flags     = DF
  frag      = 0L
  ttl       = 64
  proto     = tcp
  chksum    = 0x9afd
  src       = 100.99.98.97
  dst       = 10.9.8.7
  \options   \
###[ TCP ]###
     sport     = 2927
     dport     = www
     seq       = 0
     ack       = 0
     dataofs   = 8L
     reserved  = 0L
     flags     = S
     window    = 8192
     chksum    = 0x2c0e
     urgptr    = 0
     options   = [('Timestamp', (0, 0)), ('EOL', None)]
>>>