Saving and loading objects and using pickle

| | | |

I´m trying to save and load objects using pickle module.
First I declare my objects:

>>> class Fruits:pass
...
>>> banana = Fruits()

>>> banana.color = "yellow"
>>> banana.value = 30

After that I open a file called "Fruits.obj"(previously I created a new .txt file and I renamed "Fruits.obj"):

>>> import pickle
>>> filehandler = open(b"Fruits.obj";"wb")
>>> pickle.dump(banana,filehandler)

After do this I close my session and I began a new one and I put the next (trying to access to the object that it supposed to be saved):

file = open("Fruits.obj","r")
object_file = pickle.load(file)

But I have this message:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:Python31libpickle.py", line 1365, in load
encoding=encoding, errors=errors).load()
ValueError: read() from the underlying stream did notreturn bytes

I don´t know what to do because I don´t understand this message. Does anyone know How I can load my object "banana"? Thank you!

EDIT: As some of you have sugested I put:

>>> import pickle
>>> file = open("Fruits.obj","rb")

There were no problem, but the next I put was:

>>> object_file = pickle.load(file)

And I have error:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:Python31libpickle.py", line 1365, in load
encoding=encoding, errors=errors).load()
EOFError

Saving and loading objects and using pickle open: Questions

How can I open multiple files using "with open" in Python?

5 answers

I want to change a couple of files at one time, iff I can write to all of them. I"m wondering if I somehow can combine the multiple open calls with the with statement:

try:
  with open("a", "w") as a and open("b", "w") as b:
    do_something()
except IOError as e:
  print "Operation failed: %s" % e.strerror

If that"s not possible, what would an elegant solution to this problem look like?

788

Answer #1

As of Python 2.7 (or 3.1 respectively) you can write

with open("a", "w") as a, open("b", "w") as b:
    do_something()

In earlier versions of Python, you can sometimes use contextlib.nested() to nest context managers. This won"t work as expected for opening multiples files, though -- see the linked documentation for details.


In the rare case that you want to open a variable number of files all at the same time, you can use contextlib.ExitStack, starting from Python version 3.3:

with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for fname in filenames]
    # Do something with "files"

Most of the time you have a variable set of files, you likely want to open them one after the other, though.

open() in Python does not create a file if it doesn"t exist

5 answers

What is the best way to open a file as read/write if it exists, or if it does not, then create it and open it as read/write? From what I read, file = open("myfile.dat", "rw") should do this, right?

It is not working for me (Python 2.6.2) and I"m wondering if it is a version problem, or not supposed to work like that or what.

The bottom line is, I just need a solution for the problem. I am curious about the other stuff, but all I need is a nice way to do the opening part.

The enclosing directory was writeable by user and group, not other (I"m on a Linux system... so permissions 775 in other words), and the exact error was:

IOError: no such file or directory.

778

Answer #1

You should use open with the w+ mode:

file = open("myfile.dat", "w+")

Difference between modes a, a+, w, w+, and r+ in built-in open function?

5 answers

In the python built-in open function, what is the exact difference between the modes w, a, w+, a+, and r+?

In particular, the documentation implies that all of these will allow writing to the file, and says that it opens the files for "appending", "writing", and "updating" specifically, but does not define what these terms mean.

721

Answer #1

The opening modes are exactly the same as those for the C standard library function fopen().

The BSD fopen manpage defines them as follows:

 The argument mode points to a string beginning with one of the following
 sequences (Additional characters may follow these sequences.):

 ``r""   Open text file for reading.  The stream is positioned at the
         beginning of the file.

 ``r+""  Open for reading and writing.  The stream is positioned at the
         beginning of the file.

 ``w""   Truncate file to zero length or create text file for writing.
         The stream is positioned at the beginning of the file.

 ``w+""  Open for reading and writing.  The file is created if it does not
         exist, otherwise it is truncated.  The stream is positioned at
         the beginning of the file.

 ``a""   Open for writing.  The file is created if it does not exist.  The
         stream is positioned at the end of the file.  Subsequent writes
         to the file will always end up at the then current end of file,
         irrespective of any intervening fseek(3) or similar.

 ``a+""  Open for reading and writing.  The file is created if it does not
         exist.  The stream is positioned at the end of the file.  Subse-
         quent writes to the file will always end up at the then current
         end of file, irrespective of any intervening fseek(3) or similar.

Saving and loading objects and using pickle rename: Questions

Rename a dictionary key

5 answers

Is there a way to rename a dictionary key, without reassigning its value to a new name and removing the old name key; and without iterating through dict key/value?

In case of OrderedDict, do the same, while keeping that key"s position.

546

Answer #1

For a regular dict, you can use:

mydict[k_new] = mydict.pop(k_old)

This will move the item to the end of the dict, unless k_new was already existing in which case it will overwrite the value in-place.

For a Python 3.7+ dict where you additionally want to preserve the ordering, the simplest is to rebuild an entirely new instance. For example, renaming key 2 to "two":

>>> d = {0:0, 1:1, 2:2, 3:3}
>>> {"two" if k == 2 else k:v for k,v in d.items()}
{0: 0, 1: 1, "two": 2, 3: 3}

The same is true for an OrderedDict, where you can"t use dict comprehension syntax, but you can use a generator expression:

OrderedDict((k_new if k == k_old else k, v) for k, v in od.items())

Modifying the key itself, as the question asks for, is impractical because keys are hashable which usually implies they"re immutable and can"t be modified.

How to rename a file using Python

5 answers

I want to change a.txt to b.kml.

485

Answer #1

Use os.rename:

import os

os.rename("a.txt", "b.kml")

Rename multiple files in a directory in Python

5 answers

I"m trying to rename some files in a directory using Python.

Say I have a file called CHEESE_CHEESE_TYPE.*** and want to remove CHEESE_ so my resulting filename would be CHEESE_TYPE

I"m trying to use the os.path.split but it"s not working properly. I have also considered using string manipulations, but have not been successful with that either.

410

Answer #1

Use os.rename(src, dst) to rename or move a file or a directory.

$ ls
cheese_cheese_type.bar  cheese_cheese_type.foo
$ python
>>> import os
>>> for filename in os.listdir("."):
...  if filename.startswith("cheese_"):
...    os.rename(filename, filename[7:])
... 
>>> 
$ ls
cheese_type.bar  cheese_type.foo

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