hints

main.py

file_lines = ['a line','another one ','','  ','last','> we want this','> and this']

print("First Code said \n")

# Get lines that starts with ">" and not empty
for line in (l for l in file_lines if l.strip()  and l.startswith(">") ) :
    print(line)
    
# An empty line dosen't start with a ">" so the code can be shortened to 
print("Second code said \n")
for line in (l for l in file_lines if l.startswith(">") ) :
    print(line)


my_dict = {'a':[1,2,3]}
print(my_dict)

# Now we append a value to the 'b' key in my_dict, using setdefault before append setes the value of 'b' to an empty array if it's not already set
my_dict.setdefault('b',[]).append(3)
print(my_dict)
if 'c' not in my_dict:
    my_dict['c'] = []
    my_dict['c'].append(4)
print(my_dict)

output.txt

First Code said 

> we want this
> and thisSecond code said 

> we want this
> and this
{'a': [1, 2, 3]}
{'a': [1, 2, 3], 'b': [3]}
{'a': [1, 2, 3], 'b': [3], 'c': [4]}