一个程序，给定一个字符串，统计字母a的数量，根据这个程序设计测试用例设计

当你设计一个程序来统计一个字符串中字母"a"的数量时，你可以考虑以下Python示例代码：

def count_a(s):
    return s.count('a')

# 测试用例设计：
# 1. 非空字符串，包含多个"a":
test_case_1 = "apple"
expected_output_1 = 1
assert count_a(test_case_1) == expected_output_1, f"Test case 1 failed: Got {count_a(test_case_1)} instead of {expected_output_1}"

# 2. 空字符串，没有"a":
test_case_2 = ""
expected_output_2 = 0
assert count_a(test_case_2) == expected_output_2, f"Test case 2 failed: Got {count_a(test_case_2)} instead of {expected_output_2}"

# 3. 包含其他字符的混合字符串：
test_case_3 = "banana and cat"
expected_output_3 = 3
assert count_a(test_case_3) == expected_output_3, f"Test case 3 failed: Got {count_a(test_case_3)} instead of {expected_output_3}"

# 4. 全部由"a"组成的字符串：
test_case_4 = "aaaaa"
expected_output_4 = 5
assert count_a(test_case_4) == expected_output_4, f"Test case 4 failed: Got {count_a(test_case_4)} instead of {expected_output_4}"

#