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Undesired deletion of temporaly files

I am try to create some temporal files and make some operations on them inside a loop. Then I will access the information on all of the temporal files. And do some operations with that information. For simplicity I brought the following code that reproduces my issue:
import tempfile
tmp_files = []
for i in range(40):
tmp = tempfile.NamedTemporaryFile(suffix=".txt")
with open(tmp.name, "w") as f:
f.write(str(i))
tmp_files.append(tmp.name)
string = ""
for tmp_file in tmp_files:
with open(tmp_file, "r") as f:
data = f.read()
string += data
print(string)
ERROR:
with open(tmp_file, "r") as f: FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tmpynh0kbnw.txt'
When I look on /tmp directory (with some time.sleep(2) on the loop) I see that the file is deleted and only one is preserved. And for that the error.
Of course I could handle to keep all the files with the flag tempfile.NamedTemporaryFile(suffix=".txt", delete=False). But that is not the idea. I would like to hold the temporal files just for the running time of the script. I also could delete the files with os.remove. But my question is more why this happen. Because I expected that the files hold to the end of the running. Because I don't close the file on the execution (or do I?).
A lot of thanks in advance.

tdelaney does already answer your actual question.
I just would like to offer you an alternative to NamedTemporaryFile. Why not creating a temporary folder which is removed (with all files in it) at the end of the script?
Instead of using a NamedTemporaryFile, you could use tempfile.TemporaryDirectory. The directory will be deleted when closed.
The example below uses the with statement which closes the file handle automatically when the block ends (see John Gordon's comment).
import os
import tempfile
with tempfile.TemporaryDirectory() as temp_folder:
tmp_files = []
for i in range(40):
tmp_file = os.path.join(temp_folder, f"{i}.txt")
with open(tmp_file, "w") as f:
f.write(str(i))
tmp_files.append(tmp_file)
string = ""
for tmp_file in tmp_files:
with open(tmp_file, "r") as f:
data = f.read()
string += data
print(string)

By default, a NamedTemporaryFile deletes its file when closed. its a bit subtle, but tmp = tempfile.NamedTemporaryFile(suffix=".txt") in the loop causes the previous file to be deleted when tmp is reassigned. One option is to use the delete=False parameter. Or, just keep the file open and seek to the beginning after the write.
NamedTemporaryFile is already a file object - you can write to it directly without reopening. Just make sure the mode is "write plus" and in text, not binary mode. Put the code an a try/finally block to make sure the files are really deleted at the end.
import tempfile
tmp_files = []
try:
for i in range(40):
tmp = tempfile.NamedTemporaryFile(suffix=".txt", mode="w+")
tmp.write(str(i))
tmp.seek(0)
tmp_files.append(tmp)
string = ""
for tmp_file in tmp_files:
data = tmp_file.read()
string += data
finally:
for tmp_file in tmp_files:
tmp_file.close()
print(string)

File content not reading without seek in python

In my case I am going to write some content to a file in bytearray format and tries to read the content that I have written . But here the problem is if I am not giving the seek function then the file content read is empty. What I understood is by default the reference point is at the beginning of the file which is similar to seek(0). Please help me to understand this problem. I will give you both scenarios as example here
Without seek command
filename = "my_file"
Arr = [0x1, 0x2]
file_handle = open(filename, "wb+")
binary_format = bytearray(Arr)
file_handle.write(binary_format)
#file_handle.seek(0) #Here commenting the seek(0) part
print("file_handle-",file_handle.read())
file_handle.close()
Output in the console
file_handle- b''
With seek command
filename = "my_file"
Arr = [0x1, 0x2]
file_handle = open(filename, "wb+")
binary_format = bytearray(Arr)
file_handle.write(binary_format)
file_handle.seek(0)
print("file_handle-",file_handle.read())
file_handle.close()
Output in the console is
file_handle- b'\x01\x02'
Is the seek(0) is mandatory here even if by default it points to the beginning of file ?

Assigning column names in a txt file in python

I have the following code that extracts needed data from a xml file. I can save it to terminal, and open it with no problem. I am in trouble inserting column names to the txt file, however. I have been searching the answer but found no right solution. Would anyone help me here? Thank you!!
import sys
reload(sys)
sys.setdefaultencoding('utf-8')
orig_stdout = sys.stdout
sys.stdout = f
for program in root.findall('program'):
programID = program.find('programID')
season = program.find('season')
print programID, season
sys.stdout = orig_stdout
f.close()

The way to write data to a file in Python is to call .write(content) on the file object, not to redirect stdout to it.
Instead of all your lines messing with sys.stdout, try this:
f = open("file.txt", "w") # Open the file in (w)rite mode
f.write("programID,Season\n") # Header line
for program in root.findall("program"):
programID = program.find("programID").text
season = program.find("season").text
line = programID + "," + season + "\n"
f.write(line)
f.close() # Outside the loop
There are better ways to make the string for line, but we don't need to worry about those at the moment.

Printing a .txt File Python

I need some help Im trying to display the text files contents (foobar) with this code
text = open('C:\\Users\\Imran\\Desktop\\text.txt',"a")
rgb = text.write("foobar\n")
print (rgb)
text.close()
for some reason it keeps displaying a number. If anyone could help that would be awesome, thanks in advance
EDIT: I am Working with Python 3.3.

Print the contents of the file like this:
with open(filename) as f:
for line in f:
print(line)
Use with to ensure that the file handle will be closed when you are finished with it.
Append to the file like this:
with open(filename, 'a') as f:
f.write('some text')

# Open a file
fo = open("foo.txt", "r+")
str = fo.read();
print "Read String is : ", str
# Close opend file
fo.close()
More: http://www.tutorialspoint.com/python/python_files_io.htm

You are printing the number of written bytes. That won't work. Also you might need to open the file as RW.
Code:
text = open('...', "a")
text.write("foo\n")
text = open('...', "r")
print text.read()

If you want to display the contents of the file open it in read mode
f=open("PATH_TO_FILE", 'r')
And then print the contents of file using
for line in f:
print(line) # In Python3.
And yes, don't forget to close the file pointer f.close() after you finish the reading

Serving binary file from web server to client

Usually, when I want to transfer a web server text file to client, here is what I did
import cgi
print "Content-Type: text/plain"
print "Content-Disposition: attachment; filename=TEST.txt"
print
filename = "C:\\TEST.TXT"
f = open(filename, 'r')
for line in f:
print line
Works very fine for ANSI file. However, say, I have a binary file a.exe (This file is in web server secret path, and user shall not have direct access to that directory path). I wish to use the similar method to transfer. How I can do so?
What content-type I should use?
Using print seems to have corrupted content received at client side. What is the correct method?
I use the following code.
#!c:/Python27/python.exe -u
import cgi
print "Content-Type: application/octet-stream"
print "Content-Disposition: attachment; filename=jstock.exe"
print
filename = "C:\\jstock.exe"
f = open(filename, 'rb')
for line in f:
print line
However, when I compare the downloaded file with original file, it seems there is an extra whitespace (or more) for after every single line.

Agree with the above posters about 'rb' and Content-Type headers.
Additionally:
for line in f:
print line
This might be a problem when encountering \n or \r\n bytes in the binary file. It might be better to do something like this:
import sys
while True:
data = f.read(4096)
sys.stdout.write(data)
if not data:
break
Assuming this is running on windows in a CGI environment, you will want to start the python process with the -u argument, this will ensure stdout isn't in text-mode

When opening a file, you can use open(filename, 'rb') - the 'b' flag marks it as binary. For a general handler, you could use some form of mime magic (I'm not familiar with using it from Python, I've only ever used it from PHP a couple of years ago). For the specific case, .exe is application/octet-stream.

Content-type of .exe is tipically application/octet-stream.
You might want to read your file using open(filename, 'rb') where b means binary.
To avoid the whitespace problem, you could try with:
sys.stdout.write(open(filename,"rb").read())
sys.stdout.flush()
or even better, depending on the size of your file, use the Knio approach:
fo = open(filename, "rb")
while True:
buffer = fo.read(4096)
if buffer:
sys.stdout.write(buffer)
else:
break
fo.close()

For anyone using Windows Server 2008 or 2012 and Python 3, here's an update...
After many hours of experimentation I have found the following to work reliably:
import io
with io.open(sys.stdout.fileno(),"wb") as fout:
with open(filename,"rb") as fin:
while True:
data = fin.read(4096)
fout.write(data)
if not data:
break