"for line in file" returns letters instead of lines (python)

Question

Sometimes when I use a for loop to iterate through lines of a file, I get each letter separately instead of each line. Can someone explain why that is?

Answer 1

See iter() and Iterator Types

for a in something:
    pass 

If somethings type is a itereable, a will take all values of your iterable in turn. What exactly a becomes is hidden inside the __iter__(self): implementation of the object you are iterating over (afaik).

• If you iterate over a list, it will present you with all values in turns.
• If you iterate over a dict, it will present you with all dict.keys() in turns.
• If you iterate over a string, it will present you each character in turn.

Generators and sequences also provide iterating over, see Understanding generators in Python and Sequence Types

If you iterate over a file, you will get (textfile, not binary) it line by line, unless you call other methods on the file first ():

Demodata:

with open("f.txt","w") as f:
    f.write("""Some Text
Some text after a newline.
More text in a new line.""")

Iterate of file using it as iterable:

with open("f.txt","r") as r:
    for c in r: # returns each single line including \n (thats the way next() of file works
        print (c) # appends another \n due to default end='\n' 
print("-"*30)

Output:

Some Text

Some text after a newline.

More text in a new line.
------------------------------

---

Iterate over file.read():

with open("f.txt","r") as r:
    t = r.read() # returns a single string of the whole files content
    for c in t:  # you iterate characterwise over a string!
        print (c)
print("-"*30)

Output:

S
o
m
e

[... snipped ...]

l
i
n
e
.
------------------------------

---

Iterate over file using readline():

with open("f.txt","r") as r:
    for c in r.readline(): # returns only the first line as string and iterates over this string
        print (c)          # the other lines are omitted!
print("-"*30)

Output:

S
o
m
e

T
e
x
t

------------------------------

---

Iterate over file using readlines():

with open("f.txt","r") as r:
    for c in r.readlines(): # returns a list of strings
        print (c)  # appends another \n due to default end='\n' 
print("-"*30)

Output:

Some Text

Some text after a newline.

More text in a new line.
------------------------------

---

Iterage over binary file:

with open("f.bin","wb") as f:
    f.write(b"294827523")
    f.write(b"2927523")
    f.write(b"-27523")
    f.write(b"\n-27523")

with open("f.bin","rb") as r:
    for b in r:
        print (b)

Output:

b'2948275232927523-27523\n'
b'-27523'