Source.Python forums

#include <sourcemod>
#include <sdktools>
#include <cstrike>

#pragma semicolon 1

new String:gS_Knives[][] = 
{
    "weapon_bayonet",
    "weapon_knife_gut",
    "weapon_knife_flip",
    "weapon_knife_m9_bayonet",
    "weapon_knife_karambit"
};

public Plugin:myinfo = 
{
    name = "CS:GO knives example",
    author = "shavit",
    version = "1.0"
}

public OnPluginStart()
{
    HookEvent("item_pickup", Item_Pickup);
}

public Action:Item_Pickup(Handle:event, const String:name[], bool:dB) 
{
    new client = GetClientOfUserId(GetEventInt(event, "userid"));
    
    if(!IsValidClient(client))
    {
        return Plugin_Continue;
    }
    
    new String:weapon[32];
    GetEventString(event, "item", weapon, 32);
    
    if(StrContains(weapon, "knife", false) != -1)
    {
        new currentknife = GetPlayerWeaponSlot(client, 2);
        
        if(currentknife != INVALID_ENT_REFERENCE)
        {
            RemovePlayerItem(client, currentknife);
        }
        
        new knife = GivePlayerItem(client, gS_Knives[GetRandomInt(0, sizeof(gS_Knives))]);
        
        EquipPlayerWeapon(client, knife);
    }
    
    return Plugin_Continue;
}

// Stocks
stock bool:IsValidClient(client, bool:alive = false)
{
    return (client >= 1 && client <= MaxClients && IsClientConnected(client) && IsClientInGame(client) && (alive == false || IsPlayerAlive(client)));
}