python - How to used NamedTemporaryFile (and when is it closed?) -

i'm trying write series of functions writes temporary file, stuff written file. i'm trying understand how file dealt with.

what i'd in abstract is:

def create_function(inputs):     # create temp file, write contents  def function1(file):     # stuff temp file  def function2(file):     # other stuff temp file 

so can like:

my_file = create_function(my_inputs) function1(my_file) function2(my_file) 

so here's i've done:

def db_cds_to_fna(collection, open_file):     """     pulls data mongodb , writes temporary file - writing arbitrary string doesn't alter question (i'm pretty sure)     """     record in db[collection].find({"type": "cds"}):         open_file.write(">{}|{}|{}\n{}\n".format(             collection,             record["_id"],             record["annotation"],             record["dna_seq"]             )         )      return open_file.name  def check_file(open_file):     lines = 0     line in open_file:         if lines < 5:             print line             lines += 1         else:             break 

with code, if run following:

from tempfile import namedtemporaryfile tmp_file = namedtemporaryfile() tmp_fna =  db_cds_to_fna('test_collection', tmp_file)  check_file(tmp_file) 

this code runs, doesn't print anything. file there , written, because if run print popen(['head', tmp_fna], stdout=pipe)[0], expected beginning of file. or, if change check_file() accept tmp_file.name , with open(tmp_file.name, 'r')... inside function, works.

so question 1 - why can write tmp_file, can't read different function without re-opening it?

now, i'd have tmp_file = namedtemporaryfile() inside db_cds_to_fna() function, when try , run:

tmp_fna =  db_cds_to_fna('test_collection') check_file(tmp_file) 

i error no such file or folder

so question 2 is: there way keep temporary file around function use? know how write file specified path , delete it, suspect there's built in way , i'd learn.

you're writing file, you're attempting read end of writes. add seek before start reading, go beginning of file:

def check_file(open_file):     lines = 0     open_file.seek(0)     line in open_file:         if lines < 5:             print line             lines += 1         else:             break 

for second question, note namedtemporaryfile works temporaryfile in that:

it destroyed closed (including implicit close when object garbage collected).

if open file in function , return, file goes out of scope , closed , garbage collected. you'll need keep reference file object alive in order keep being collected. can returning file object function (and making sure assign something). here's trivial example:

def mycreate():     return namedtemporaryfile() def mywrite(f, i):     f.write(i) def myread(f):     f.seek(0)     return f.read()  f = mycreate()        # 'f' reference file created in function,                        # keep being garbage collected mywrite(f, b'hi') myread(f)