For file I/O what is the purpose of:
with open
and should I use it instead of:
f=open('file', 'w')
f.write('foo)'
f.close()
Always use the with statement.
From docs:
It is good practice to use the with keyword when dealing with file
objects. This has the advantage that the file is properly closed after
its suite finishes, even if an exception is raised on the way. It is also much shorter than writing equivalent try-finally blocks.
If you don't close the file explicitly then the file object may hang around in the memory until it is garbage collected, which implicitly calls close() on the file object. So, better use the with statement, as it will close the file explicitly even if an error occurs.
Related: Does a File Object Automatically Close when its Reference Count Hits Zero?
Yes. You should use with whenever possible.
This is using the return value of open as a context manager. Thus with is used not just specifically for open, but it should be preferred in any case that some cleanup needs to occur with regards to the object (that you would normally put in a finally block). In this case: on exiting the context, the .close() method of the file object is invoked.
Another good example of a context manager "cleaning up" is threading's Lock:
lock = Lock()
with lock:
#do thing
#lock is released outside the context
In this case, the context manager is .release()-ing the lock.
Anything with an __enter__ and __exit__ method can be used as a context manager. Or, better, you can use contextlib to make context managers with the #contextmanager decoration. More here.
Basically what it is trying to avoid is this:
set things up
try:
do something
finally:
tear things down
but with the with statement you can safely, say open a file and as soon as you exit the scope of the with statement the file will be closed.
The with statement calls the __enter__ function of a class, which does your initial set up and it makes sure it calls the __exit__ function at the end, which makes sure that everything is closed properly.
The with statement is a shortcut for easily writing more robust code. This:
with open('file', 'w') as f:
f.write('foo')
is equivalent to this:
try:
f = open('file', 'w')
f.write('foo')
finally:
f.close()
Related
Is it correctly understood that the following two functions do the exact same? No matter how they are invoked.
def test():
file = open("testfile.txt", "w")
file.write("Hello World")
def test_2():
with open("testfile.txt", "w") as f:
f.write("Hello World")
Since python invokes the close method when an object is no longer referenced.
If not then this quote confuses me:
Python automatically closes a file when the reference object of a file
is reassigned to another file. It is a good practice to use the
close() method to close a file.
from https://www.tutorialspoint.com/python/file_close.htm
No, the close method would be invoked by python garbage collector (finalizer) machinery in the first case, and immediately in the second case. If you loop calling your test or test_2 functions thousands of times, the observed behavior could be different.
File descriptors are (at least on Linux) a precious and scarce resource (when it is exhausted, the open(2) syscall fails). On Linux use getrlimit(2) with RLIMIT_NOFILE to query the limit on the number of file descriptors for your process. You should prefer the close(2) syscall to be invoked quickly once a file handle is useless.
Your question is implementation specific, operating system specific, and computer specific. You may want to understand more about operating systems by reading Operating Systems: Three Easy Pieces.
On Linux, try also the cat /proc/$$/limits or cat /proc/self/limits command in a terminal. You would see a line starting with Max open files (on my Debian desktop computer, right now in december 2019, the soft limit is 1024). See proc(5).
No. The first one will not save the information correctly. You need to use file.close() to ensure that file is closed properly and data is saved.
On the other hand, with statement handles file operations for you. It will keep the file open for as long as the program keeps executing at the same indent level and as soon as it goes to a level higher will automatically close and save the file.
More information here.
In case of test function, close method is not called until Python garbage collector will del f, in this case it's invoked by file __del__ magic method which is invoked on variable deletion.
In case of test_2 function, close method is called when code execution goes outside of with statement. Read more about python context managers which is used by with statement.
with foo as f:
do_something()
roughly is just syntax sugar for:
f = foo.__enter__()
do_something()
f.__exit__()
and in case of file, __exit__ implicitly calls close
No, it is not correctly understood. The close method is invoked via the __exit__ method, which is only invoked when exiting a with statement not when exiting a function. Se code example below:
class Temp:
def __exit__(self, exc_type, exc_value, tb):
print('exited')
def __enter__(self):
pass
def make_temp():
temp = Temp()
make_temp()
print('temp_make')
with Temp() as temp:
pass
print('temp_with')
Witch outputs:
temp_make
exited
temp_with
For writing large files in gridFS using put(), is it necessary to use context manager with ?
Looking at the documentation for put() here, calling put() is equivalent to doing,
try:
f = new_file(**kwargs)
f.write(data)
finally:
f.close()
Does that mean open and close for file is done automatically and hence do not require without explicit need to?
gridfs.GridFS.put isn't a context manager. It doesn't define __enter__ and __exit__ methods of the context management protocol.
Using it directly without some modification as a context manager will result in an AttributeError.
Using gridfs.GridFS.put as-is saves you few lines of code and more importantly having to manage opening and closing the GridFile.
I think this question is more of a "coding style" rather than technical issue.
Said I have a line of code:
buf = open('test.txt','r').readlines()
...
Will the file descriptor automatically close, or will it stay in the memory?
If the file descriptor is not closed, what is the prefer way to close it?
If you assign the file object to a variable, you can explicitly close it using .close()
f = open('test.txt','r')
buf = f.readlines()
f.close()
Alternatively (and more generally preferred), you can use the with keyword (Python 2.5 and greater) as mentioned in the Python docs:
It is good practice to use the with
keyword when dealing with file
objects. This has the advantage that
the file is properly closed after its
suite finishes, even if an exception
is raised on the way. It is also much
shorter than writing equivalent
try-finally blocks:
>>> with open('test.txt','r') as f:
... buf = f.readlines()
>>> f.closed
True
Usually in CPython, the file is closed right away when the reference count drops to zero (although this behaviour is not guaranteed for future versions of CPython)
In other implementations, such as Jython, the file won't be closed until it is garbarge collected, which can be a long time later.
It's poor style to have code that works differently depending on the implementation's behaviour.
If it's just for a quickie script or something you are trying in the interpreter shell it's good enough, but for any sort of production work you should usually use a context manager as in Falmarri's answer
It will stay in memory until the garbage collector closes it. You should always explicitly close your file descriptors. Just do something like this:
with open('test.txt', 'r') as f:
buf = f.readlines()
It will be automatically closed, but it depends on implementation exactly when. It's nicer to explicitly use a with-block, but if you are just writing a small script for yourself that you run occasionally it doesn't really matter.
You can also try using os.close(fd) method.
I came across the Python with statement for the first time today. I've been using Python lightly for several months and didn't even know of its existence! Given its somewhat obscure status, I thought it would be worth asking:
What is the Python with statement
designed to be used for?
What do
you use it for?
Are there any
gotchas I need to be aware of, or
common anti-patterns associated with
its use? Any cases where it is better use try..finally than with?
Why isn't it used more widely?
Which standard library classes are compatible with it?
I believe this has already been answered by other users before me, so I only add it for the sake of completeness: the with statement simplifies exception handling by encapsulating common preparation and cleanup tasks in so-called context managers. More details can be found in PEP 343. For instance, the open statement is a context manager in itself, which lets you open a file, keep it open as long as the execution is in the context of the with statement where you used it, and close it as soon as you leave the context, no matter whether you have left it because of an exception or during regular control flow. The with statement can thus be used in ways similar to the RAII pattern in C++: some resource is acquired by the with statement and released when you leave the with context.
Some examples are: opening files using with open(filename) as fp:, acquiring locks using with lock: (where lock is an instance of threading.Lock). You can also construct your own context managers using the contextmanager decorator from contextlib. For instance, I often use this when I have to change the current directory temporarily and then return to where I was:
from contextlib import contextmanager
import os
#contextmanager
def working_directory(path):
current_dir = os.getcwd()
os.chdir(path)
try:
yield
finally:
os.chdir(current_dir)
with working_directory("data/stuff"):
# do something within data/stuff
# here I am back again in the original working directory
Here's another example that temporarily redirects sys.stdin, sys.stdout and sys.stderr to some other file handle and restores them later:
from contextlib import contextmanager
import sys
#contextmanager
def redirected(**kwds):
stream_names = ["stdin", "stdout", "stderr"]
old_streams = {}
try:
for sname in stream_names:
stream = kwds.get(sname, None)
if stream is not None and stream != getattr(sys, sname):
old_streams[sname] = getattr(sys, sname)
setattr(sys, sname, stream)
yield
finally:
for sname, stream in old_streams.iteritems():
setattr(sys, sname, stream)
with redirected(stdout=open("/tmp/log.txt", "w")):
# these print statements will go to /tmp/log.txt
print "Test entry 1"
print "Test entry 2"
# back to the normal stdout
print "Back to normal stdout again"
And finally, another example that creates a temporary folder and cleans it up when leaving the context:
from tempfile import mkdtemp
from shutil import rmtree
#contextmanager
def temporary_dir(*args, **kwds):
name = mkdtemp(*args, **kwds)
try:
yield name
finally:
shutil.rmtree(name)
with temporary_dir() as dirname:
# do whatever you want
I would suggest two interesting lectures:
PEP 343 The "with" Statement
Effbot Understanding Python's
"with" statement
1.
The with statement is used to wrap the execution of a block with methods defined by a context manager. This allows common try...except...finally usage patterns to be encapsulated for convenient reuse.
2.
You could do something like:
with open("foo.txt") as foo_file:
data = foo_file.read()
OR
from contextlib import nested
with nested(A(), B(), C()) as (X, Y, Z):
do_something()
OR (Python 3.1)
with open('data') as input_file, open('result', 'w') as output_file:
for line in input_file:
output_file.write(parse(line))
OR
lock = threading.Lock()
with lock:
# Critical section of code
3.
I don't see any Antipattern here.
Quoting Dive into Python:
try..finally is good. with is better.
4.
I guess it's related to programmers's habit to use try..catch..finally statement from other languages.
The Python with statement is built-in language support of the Resource Acquisition Is Initialization idiom commonly used in C++. It is intended to allow safe acquisition and release of operating system resources.
The with statement creates resources within a scope/block. You write your code using the resources within the block. When the block exits the resources are cleanly released regardless of the outcome of the code in the block (that is whether the block exits normally or because of an exception).
Many resources in the Python library that obey the protocol required by the with statement and so can used with it out-of-the-box. However anyone can make resources that can be used in a with statement by implementing the well documented protocol: PEP 0343
Use it whenever you acquire resources in your application that must be explicitly relinquished such as files, network connections, locks and the like.
Again for completeness I'll add my most useful use-case for with statements.
I do a lot of scientific computing and for some activities I need the Decimal library for arbitrary precision calculations. Some part of my code I need high precision and for most other parts I need less precision.
I set my default precision to a low number and then use with to get a more precise answer for some sections:
from decimal import localcontext
with localcontext() as ctx:
ctx.prec = 42 # Perform a high precision calculation
s = calculate_something()
s = +s # Round the final result back to the default precision
I use this a lot with the Hypergeometric Test which requires the division of large numbers resulting form factorials. When you do genomic scale calculations you have to be careful of round-off and overflow errors.
An example of an antipattern might be to use the with inside a loop when it would be more efficient to have the with outside the loop
for example
for row in lines:
with open("outfile","a") as f:
f.write(row)
vs
with open("outfile","a") as f:
for row in lines:
f.write(row)
The first way is opening and closing the file for each row which may cause performance problems compared to the second way with opens and closes the file just once.
See PEP 343 - The 'with' statement, there is an example section at the end.
... new statement "with" to the Python
language to make
it possible to factor out standard uses of try/finally statements.
points 1, 2, and 3 being reasonably well covered:
4: it is relatively new, only available in python2.6+ (or python2.5 using from __future__ import with_statement)
The with statement works with so-called context managers:
http://docs.python.org/release/2.5.2/lib/typecontextmanager.html
The idea is to simplify exception handling by doing the necessary cleanup after leaving the 'with' block. Some of the python built-ins already work as context managers.
Another example for out-of-the-box support, and one that might be a bit baffling at first when you are used to the way built-in open() behaves, are connection objects of popular database modules such as:
sqlite3
psycopg2
cx_oracle
The connection objects are context managers and as such can be used out-of-the-box in a with-statement, however when using the above note that:
When the with-block is finished, either with an exception or without, the connection is not closed. In case the with-block finishes with an exception, the transaction is rolled back, otherwise the transaction is commited.
This means that the programmer has to take care to close the connection themselves, but allows to acquire a connection, and use it in multiple with-statements, as shown in the psycopg2 docs:
conn = psycopg2.connect(DSN)
with conn:
with conn.cursor() as curs:
curs.execute(SQL1)
with conn:
with conn.cursor() as curs:
curs.execute(SQL2)
conn.close()
In the example above, you'll note that the cursor objects of psycopg2 also are context managers. From the relevant documentation on the behavior:
When a cursor exits the with-block it is closed, releasing any resource eventually associated with it. The state of the transaction is not affected.
In python generally “with” statement is used to open a file, process the data present in the file, and also to close the file without calling a close() method. “with” statement makes the exception handling simpler by providing cleanup activities.
General form of with:
with open(“file name”, “mode”) as file_var:
processing statements
note: no need to close the file by calling close() upon file_var.close()
The answers here are great, but just to add a simple one that helped me:
with open("foo.txt") as file:
data = file.read()
open returns a file
Since 2.6 python added the methods __enter__ and __exit__ to file.
with is like a for loop that calls __enter__, runs the loop once and then calls __exit__
with works with any instance that has __enter__ and __exit__
a file is locked and not re-usable by other processes until it's closed, __exit__ closes it.
source: http://web.archive.org/web/20180310054708/http://effbot.org/zone/python-with-statement.htm
I have read that when file is opened using the below format
with open(filename) as f:
#My Code
f.close()
explicit closing of file is not required . Can someone explain why is it so ? Also if someone does explicitly close the file, will it have any undesirable effect ?
The mile-high overview is this: When you leave the nested block, Python automatically calls f.close() for you.
It doesn't matter whether you leave by just falling off the bottom, or calling break/continue/return to jump out of it, or raise an exception; no matter how you leave that block. It always knows you're leaving, so it always closes the file.*
One level down, you can think of it as mapping to the try:/finally: statement:
f = open(filename)
try:
# My Code
finally:
f.close()
One level down: How does it know to call close instead of something different?
Well, it doesn't really. It actually calls special methods __enter__ and __exit__:
f = open()
f.__enter__()
try:
# My Code
finally:
f.__exit__()
And the object returned by open (a file in Python 2, one of the wrappers in io in Python 3) has something like this in it:
def __exit__(self):
self.close()
It's actually a bit more complicated than that last version, which makes it easier to generate better error messages, and lets Python avoid "entering" a block that it doesn't know how to "exit".
To understand all the details, read PEP 343.
Also if someone does explicitly close the file, will it have any undesirable effect ?
In general, this is a bad thing to do.
However, file objects go out of their way to make it safe. It's an error to do anything to a closed file—except to close it again.
* Unless you leave by, say, pulling the power cord on the server in the middle of it executing your script. In that case, obviously, it never gets to run any code, much less the close. But an explicit close would hardly help you there.
Closing is not required because the with statement automatically takes care of that.
Within the with statement the __enter__ method on open(...) is called and as soon as you go out of that block the __exit__ method is called.
So closing it manually is just futile since the __exit__ method will take care of that automatically.
As for the f.close() after, it's not wrong but useless. It's already closed so it won't do anything.
Also see this blogpost for more info about the with statement: http://effbot.org/zone/python-with-statement.htm