Two sprites spotting each other

Hello again Stack Overflow. you probably remember me from my unit spawning problem in my pygame program, "Table Wars." I decided to change the scope of my game to a real-time strategy rather than a turn-based game. I want the game to play along the lines of top Flash game: "Age of War." Almost everything works in the game: spawning units, the HUD for the game, and even base health. Unfortunately, I can't seem to figure out how to implement the ability for units to attack enemies or the enemy base. Here is the concepts going on for the units themselves:

The unit spawns on a keypress around the team's base: K_1 spawns a sprite from the Red_Infantry class
The unit, when spawned, is added to a Group class. There are two Groups, one for each team.
The unit moves via a move_ip call within a def update until it reaches a point close to the enemy base, where it stops.

Here is how I want combat to go for the different units:

The unit stops whenever it spots an enemy within it's attack range. Different units have different attack ranges
The unit then attacks in one-second intervals.
If the unit sucessfully reduces the enemy unit's health to 0, the enemy dies, and the other may continue
This cycle repeats until the unit reaches the enemy base, where it will then attack the enemy base on one-second intervals. One of the units will be able to deal triple the normal damage to the base.

Here is a sample of my code, showing the Red_Infantry class:

class Red_Infantry(pygame.sprite.Sprite):
def __init__(self, screen):
    pygame.sprite.Sprite.__init__(self)
    self.image, self.rect = load_image('Soldier_red.png', -1)
    self.rect.move_ip(random.randint(75, 100), random.randint(275, 325))
    self.selected = 0
    self.area = screen.get_rect()
    self.health = 100 #Soldiers are have mediocre toughness.
    self.attack_damage = 25 #The amount of damage it deals
    self.range = 20 #The attack range of the unit.
    self.update()
def update(self):
    self.rect.move_ip(1, 0)
    if self.rect.right >= 725: #This position is close to the enemy base...
        self.rect.right = 725 #...where it will then stop
    if self.health <= 0:  
        self.kill() #a simple code that kills the sprite if his health reaches 0

The main loop only contains the ability to spawn each of the units.

Solution

Here's a starting point

class RedInfantry(pygame.sprite.Sprite):
    def __init__(self):
        self.screen = pygame.display.get_surface()
        self.image, self.rect = load_image('Soldier_red.png', -1)    
        self.rect.move_ip(random.randint(75, 100), random.randint(275, 325))
        self.target = None
        self.range = 20
        self.attack_cooldown = 200
        self.attack_last = 0

    def move_toward(self, target):
        # move toward players click destination, if one is set.
        # else toward my attack target

    def update(self):
        # move...
        self.find_target()
        self.move_toward(self.target)
        self.attack()
        # check HP
        if self.health <= 0:
            self.kill()

    def find_target(self):
        """finds new targets in range:
        for speed: only call this once every 200ms."""
        if self.target is not None: return
        for enemy in B.sprites():
            if distance(self.rect.center, enemy.rect.center) <= self.range:
                self.target = enemy
                return
        # else no targets in range
        self.target = None  


    def attack(self):
        """attack, if able.
        target exists? still alive? gun cooldown good?"""
        if self.target is None: return
        if self.target.health <= 0: return
        if not self.cooldown_ready(): return

        # all good, fire. could verify still in range. 
        self.target.damage(self.attack_damage)

    def cooldown_ready(self):
        # gun ready to fire? has cooldown in MS elapsed.
        now = pygame.time.get_ticks()
        if now - self.attack_last >= self.attack_cooldown:
            self.attack_last = now
            return True
        return False