Why can't I call read() twice on an open file?
Question:
For an exercise I’m doing, I’m trying to read the contents of a given file twice using the read() method. Strangely, when I call it the second time, it doesn’t seem to return the file content as a string?
Here’s the code
f = f.open()
# get the year
match = re.search(r'Popularity in (d+)', f.read())
if match:
print match.group(1)
# get all the names
matches = re.findall(r'<td>(d+)</td><td>(w+)</td><td>(w+)</td>', f.read())
if matches:
# matches is always None
Of course I know that this is not the most efficient or best way, this is not the point here. The point is, why can’t I call read() twice? Do I have to reset the file handle? Or close / reopen the file in order to do that?
Answers:
Calling read() reads through the entire file and leaves the read cursor at the end of the file (with nothing more to read). If you are looking to read a certain number of lines at a time you could use readline(), readlines() or iterate through lines with for line in handle:.
To answer your question directly, once a file has been read, with read() you can use seek(0) to return the read cursor to the start of the file (docs are here). If you know the file isn’t going to be too large, you can also save the read() output to a variable, using it in your findall expressions.
Ps. Don’t forget to close the file after you are done with it.
The read pointer moves to after the last read byte/character. Use the seek() method to rewind the read pointer to the beginning.
Every open file has an associated position.
When you read() you read from that position.
For example read(10) reads the first 10 bytes from a newly opened file, then another read(10) reads the next 10 bytes.
read() without arguments reads all of the contents of the file, leaving the file position at the end of the file. Next time you call read() there is nothing to read.
You can use seek to move the file position. Or probably better in your case would be to do one read() and keep the result for both searches.
Everyone who has answered this question so far is absolutely right – read() moves through the file, so after you’ve called it, you can’t call it again.
What I’ll add is that in your particular case, you don’t need to seek back to the start or reopen the file, you can just store the text that you’ve read in a local variable, and use it twice, or as many times as you like, in your program:
f = f.open()
text = f.read() # read the file into a local variable
# get the year
match = re.search(r'Popularity in (d+)', text)
if match:
print match.group(1)
# get all the names
matches = re.findall(r'<td>(d+)</td><td>(w+)</td><td>(w+)</td>', text)
if matches:
# matches will now not always be None
read() consumes. So, you could reset the file, or seek to the start before re-reading. Or, if it suites your task, you can use read(n) to consume only n bytes.
As other answers suggested, you should use seek().
I’ll just write an example:
>>> a = open('file.txt')
>>> a.read()
#output
>>> a.seek(0)
>>> a.read()
#same output
I always find the read method something of a walk down a dark alley. You go down a bit and stop but if you are not counting your steps you are not sure how far along you are. Seek gives the solution by repositioning, the other option is Tell which returns the position along the file. May be the Python file api can combine read and seek into a read_from(position,bytes) to make it simpler – till that happens you should read this page.
For an exercise I’m doing, I’m trying to read the contents of a given file twice using the read() method. Strangely, when I call it the second time, it doesn’t seem to return the file content as a string?
Here’s the code
f = f.open()
# get the year
match = re.search(r'Popularity in (d+)', f.read())
if match:
print match.group(1)
# get all the names
matches = re.findall(r'<td>(d+)</td><td>(w+)</td><td>(w+)</td>', f.read())
if matches:
# matches is always None
Of course I know that this is not the most efficient or best way, this is not the point here. The point is, why can’t I call read() twice? Do I have to reset the file handle? Or close / reopen the file in order to do that?
Calling read() reads through the entire file and leaves the read cursor at the end of the file (with nothing more to read). If you are looking to read a certain number of lines at a time you could use readline(), readlines() or iterate through lines with for line in handle:.
To answer your question directly, once a file has been read, with read() you can use seek(0) to return the read cursor to the start of the file (docs are here). If you know the file isn’t going to be too large, you can also save the read() output to a variable, using it in your findall expressions.
Ps. Don’t forget to close the file after you are done with it.
The read pointer moves to after the last read byte/character. Use the seek() method to rewind the read pointer to the beginning.
Every open file has an associated position.
When you read() you read from that position.
For example read(10) reads the first 10 bytes from a newly opened file, then another read(10) reads the next 10 bytes.
read() without arguments reads all of the contents of the file, leaving the file position at the end of the file. Next time you call read() there is nothing to read.
You can use seek to move the file position. Or probably better in your case would be to do one read() and keep the result for both searches.
Everyone who has answered this question so far is absolutely right – read() moves through the file, so after you’ve called it, you can’t call it again.
What I’ll add is that in your particular case, you don’t need to seek back to the start or reopen the file, you can just store the text that you’ve read in a local variable, and use it twice, or as many times as you like, in your program:
f = f.open()
text = f.read() # read the file into a local variable
# get the year
match = re.search(r'Popularity in (d+)', text)
if match:
print match.group(1)
# get all the names
matches = re.findall(r'<td>(d+)</td><td>(w+)</td><td>(w+)</td>', text)
if matches:
# matches will now not always be None
read() consumes. So, you could reset the file, or seek to the start before re-reading. Or, if it suites your task, you can use read(n) to consume only n bytes.
As other answers suggested, you should use seek().
I’ll just write an example:
>>> a = open('file.txt')
>>> a.read()
#output
>>> a.seek(0)
>>> a.read()
#same output
I always find the read method something of a walk down a dark alley. You go down a bit and stop but if you are not counting your steps you are not sure how far along you are. Seek gives the solution by repositioning, the other option is Tell which returns the position along the file. May be the Python file api can combine read and seek into a read_from(position,bytes) to make it simpler – till that happens you should read this page.