Append and Truncating together in Python -


so, @ exercise 16 of zed shaw's python book.

i thought of trying out both append , truncate on same file. know, not make sense. but, new , trying learn happen if used both.

so, first opening file in append mode , truncating , writing it.

but, truncate not working here , whatever write gets appended file.

so, can kindly explain why truncate not work ? though opening file in append mode first, believe calling truncate function after , should have worked !!!

following code:

from sys import argv  script, filename = argv  print "we're going erase %r." %filename print "if don't want that. hit ctrl-c (^c)." print "if want that, hit return."  raw_input("?")  print "opening file..." target = open(filename, 'a')  print "truncating file. goodbye!" target.truncate()  print "now i'm going ask 3 lines."  line1 = raw_input("line 1: ") line2 = raw_input("line 2: ") line3 = raw_input("line 3: ")  print "i'm going write these file."   target.write(line1 + "\n" + line2 + "\n" + line3)  print "and finally, close it." target.close()

truncate file’s size. if optional size argument present, file truncated (at most) size. size defaults current position.

when open file 'a' mode, position @ end of file.

you can this

f = open('myfile', 'a') f.tell()  # show position of cursor # can see, position @ end f.seek(0, 0) # put position @ begining f.truncate() # works !! f.close()

Comments

Post a Comment

Popular posts from this blog

jOOQ update returning clause with Oracle -

according http://www.jooq.org/doc/3.8/manual/sql-building/sql-statements/update-statement/ jooq doesn't support returning clause in update statements databases except firebird , postgres. does know, still correct oracle jooq? according oracle's documentation, oracle db supports returning clase delete, insert, or update statements. incorrect manual you're right - manual wrong , should fixed: https://github.com/jooq/jooq/issues/5470 single row update returning jooq 3.8 added support single row update .. returning statements. internally, uses callablestatement , pl/sql block of form: begin update "my_table" set "my_table"."column" = 'xyz' "my_table"."id" = 1 returning "my_table"."id", "my_table"."column" ?, ?; ? := sql%rowcount; end; the relevant issue is: https://github.com/jooq/jooq/issues/5190 multi row update returning a...
Read more

java - Warning equals/hashCode on @Data annotation lombok with inheritance -

i have entity inherits other. on other hand, i'm using lombok project reduce boilerplate code, put @data annotation. annotation @data inheritance produces next warning: generating equals/hashcode implementation without call superclass, though class not extend java.lang.object. if intentional, add '@equalsandhashcode(callsuper=false)' type. is advisable add annotation @equalsandhashcode (callsuper = true) or @equalsandhashcode (callsuper = false) ? if not added, 1 callsuper=false or callsuper=true ? the default value false . 1 if don't specify , ignore warning. yes, recommended add @equalsandhashcode annotation on @data annotated classes extend else object. cannot tell if need true or false , depends on class hierarchy, , need examined on case-by-case basis. however, project or package, can configure in lombok.config call super methods if not direct subclass of object. lombok.equalsandhashcode.callsuper = call for configuration system ...
Read more

java - BasicPathUsageException: Cannot join to attribute of basic type -

i using java 8 jpa (hibernate 5.2.1). have clause works perfectly, until introduce clause make use foreign key table values too. i getting following error: basicpathusageexception: cannot join attribute of basic type i think problem related fact have join table, , not sure how create join using join table. employee - employee_category - category model (employee.java): @manytomany(cascade = cascadetype.all, fetch=fetchtype.eager) @jointable ( name="employee_category", joincolumns={ @joincolumn(name="emp_id", referencedcolumnname="id") }, inversejoincolumns={ @joincolumn(name="cat_id", referencedcolumnname="id") } ) private set<category> categories; model (category.java): @id private string id; jpa when introduce following code, fails error above (i think problem new piece of code): join<t, category> category = from.join("id"); predicates.add(criteriabuilder.like(category....
Read more