After looking all over the Internet, I've come to this.
Let's say I have already made a text file that reads:
Hello World
Well, I want to remove the very last character (in this case d) from this text file.
So now the text file should look like this: Hello Worl
But I have no idea how to do this.
All I want, more or less, is a single backspace function for text files on my HDD.
This needs to work on Linux as that's what I'm using.
Use fileobject.seek() to seek 1 position from the end, then use file.truncate() to remove the remainder of the file:
import os
with open(filename, 'rb+') as filehandle:
filehandle.seek(-1, os.SEEK_END)
filehandle.truncate()
This works fine for single-byte encodings. If you have a multi-byte encoding (such as UTF-16 or UTF-32) you need to seek back enough bytes from the end to account for a single codepoint.
For variable-byte encodings, it depends on the codec if you can use this technique at all. For UTF-8, you need to find the first byte (from the end) where bytevalue & 0xC0 != 0x80 is true, and truncate from that point on. That ensures you don't truncate in the middle of a multi-byte UTF-8 codepoint:
with open(filename, 'rb+') as filehandle:
# move to end, then scan forward until a non-continuation byte is found
filehandle.seek(-1, os.SEEK_END)
while filehandle.read(1) & 0xC0 == 0x80:
# we just read 1 byte, which moved the file position forward,
# skip back 2 bytes to move to the byte before the current.
filehandle.seek(-2, os.SEEK_CUR)
# last read byte is our truncation point, move back to it.
filehandle.seek(-1, os.SEEK_CUR)
filehandle.truncate()
Note that UTF-8 is a superset of ASCII, so the above works for ASCII-encoded files too.
Accepted answer of Martijn is simple and kind of works, but does not account for text files with:
UTF-8 encoding containing non-English characters (which is the default encoding for text files in Python 3)
one newline character at the end of the file (which is the default in Linux editors like vim or gedit)
If the text file contains non-English characters, neither of the answers provided so far would work.
What follows is an example, that solves both problems, which also allows removing more than one character from the end of the file:
import os
def truncate_utf8_chars(filename, count, ignore_newlines=True):
"""
Truncates last `count` characters of a text file encoded in UTF-8.
:param filename: The path to the text file to read
:param count: Number of UTF-8 characters to remove from the end of the file
:param ignore_newlines: Set to true, if the newline character at the end of the file should be ignored
"""
with open(filename, 'rb+') as f:
last_char = None
size = os.fstat(f.fileno()).st_size
offset = 1
chars = 0
while offset <= size:
f.seek(-offset, os.SEEK_END)
b = ord(f.read(1))
if ignore_newlines:
if b == 0x0D or b == 0x0A:
offset += 1
continue
if b & 0b10000000 == 0 or b & 0b11000000 == 0b11000000:
# This is the first byte of a UTF8 character
chars += 1
if chars == count:
# When `count` number of characters have been found, move current position back
# with one byte (to include the byte just checked) and truncate the file
f.seek(-1, os.SEEK_CUR)
f.truncate()
return
offset += 1
How it works:
Reads only the last few bytes of a UTF-8 encoded text file in binary mode
Iterates the bytes backwards, looking for the start of a UTF-8 character
Once a character (different from a newline) is found, return that as the last character in the text file
Sample text file - bg.txt:
Здравей свят
How to use:
filename = 'bg.txt'
print('Before truncate:', open(filename).read())
truncate_utf8_chars(filename, 1)
print('After truncate:', open(filename).read())
Outputs:
Before truncate: Здравей свят
After truncate: Здравей свя
This works with both UTF-8 and ASCII encoded files.
In case you are not reading the file in binary mode, where you have only 'w' permissions, I can suggest the following.
f.seek(f.tell() - 1, os.SEEK_SET)
f.write('')
In this code above, f.seek() will only accept f.tell() b/c you do not have 'b' access. then you can set the cursor to the starting of the last element. Then you can delete the last element by an empty string.
with open(urfile, 'rb+') as f:
f.seek(0,2) # end of file
size=f.tell() # the size...
f.truncate(size-1) # truncate at that size - how ever many characters
Be sure to use binary mode on windows since Unix file line ending many return an illegal or incorrect character count.
with open('file.txt', 'w') as f:
f.seek(0, 2) # seek to end of file; f.seek(0, os.SEEK_END) is legal
f.seek(f.tell() - 2, 0) # seek to the second last char of file; f.seek(f.tell()-2, os.SEEK_SET) is legal
f.truncate()
subject to what last character of the file is, could be newline (\n) or anything else.
This may not be optimal, but if the above approaches don't work out, you could do:
with open('myfile.txt', 'r') as file:
data = file.read()[:-1]
with open('myfile.txt', 'w') as file:
file.write(data)
The code first opens the file, and then copies its content (with the exception of the last character) to the string data. Afterwards, the file is truncated to zero length (i.e. emptied), and the content of data is saved to the file, with the same name.
This is basically the same as vins ms's answer, except that it doesn't use the os package, and that is used the safer 'with open' syntax. This may not be recommended if the text file is huge. (I wrote this since none of the above approaches worked out too well for me in python 3.8).
here is a dirty way (erase & recreate)...
i don't advice to use this, but, it's possible to do like this ..
x = open("file").read()
os.remove("file")
open("file").write(x[:-1])
On a Linux system or (Cygwin under Windows). You can use the standard truncate command. You can reduce or increase the size of your file with this command.
In order to reduce a file by 1G the command would be truncate -s 1G filename. In the following example I reduce a file called update.iso by 1G.
Note that this operation took less than five seconds.
chris#SR-ENG-P18 /cygdrive/c/Projects
$ stat update.iso
File: update.iso
Size: 30802968576 Blocks: 30081024 IO Block: 65536 regular file
Device: ee6ddbceh/4000177102d Inode: 19421773395035112 Links: 1
Access: (0664/-rw-rw-r--) Uid: (1052727/ chris) Gid: (1049089/Domain Users)
Access: 2020-06-12 07:39:00.572940600 -0400
Modify: 2020-06-12 07:39:00.572940600 -0400
Change: 2020-06-12 07:39:00.572940600 -0400
Birth: 2020-06-11 13:31:21.170568000 -0400
chris#SR-ENG-P18 /cygdrive/c/Projects
$ truncate -s -1G update.iso
chris#SR-ENG-P18 /cygdrive/c/Projects
$ stat update.iso
File: update.iso
Size: 29729226752 Blocks: 29032448 IO Block: 65536 regular file
Device: ee6ddbceh/4000177102d Inode: 19421773395035112 Links: 1
Access: (0664/-rw-rw-r--) Uid: (1052727/ chris) Gid: (1049089/Domain Users)
Access: 2020-06-12 07:42:38.335782800 -0400
Modify: 2020-06-12 07:42:38.335782800 -0400
Change: 2020-06-12 07:42:38.335782800 -0400
Birth: 2020-06-11 13:31:21.170568000 -0400
The stat command tells you lots of info about a file including its size.
I am facing a problem with python (2.7) "strings".
I am using a LevelDB and want to load an image from the database and save it to disk (e.g., as test.jpeg).
To access the db I am using the plyvel package. I get the img data as follows:
img = db.get(str(id))
The problem is that LevelDB stores every value as byte string. Thus, db.get() returns a byte string. If I try to write the img to disk
f = io.open('test.jpeg', 'wb')
f.write(img)
f.close()
and open it I afterwards I get something like:
Not a JPEG file: starts with 0xc3 0xbf
c3 bf is the UTF-8 encoding of ff in latin-1, which are the first bytes of JFIF's start of image marker.
How can I encode/decode the string so taht I can open the jpeg file?
I tried bytearray(img) which did not work. Encoding /decoding the img string did not help at all.
=== EDIT ===
I am using now this sequence:
img = db.get(str(id))
img_decode_utf8 = img.decode('utf-8')
img_bytes = array.array('u', img_decode_utf8)
If I print img_bytes I get:
array('u', u'\xff\xd8\xff\xe0\x00\x10JFIF\x00\x01\x01\x00\x00\...
However, if I write it to disk, each byte is "zero extended". Hexdump:
00ff 0000 00d8 0000 00ff 0000 00e0 0000
Thanks in advance
This solved my problem:
img = db.get(str(hash_value))
img_bytes = img.decode('utf-8').encode('latin-1')
with open('test.jpeg', 'wb') as new_file:
new_file.write(img_bytes)
I am quite sure that I tried this and it did not work in the first place..
I have a file CreateTable.sql. When I do :
file --mime-encoding CreateTable.sql
The result is :
CreateTable.sql: utf-8
The command :
lot of things ...
UNICODE, UNICODEBIG, UNICODELITTLE, US-ASCII, US, UTF-7, UTF-8, UTF-16,
UTF-16BE, UTF-16LE, UTF-32, UTF-32BE, UTF-32LE, UTF7, UTF8, UTF16, UTF16BE,
UTF16LE, UTF32, UTF32BE, UTF32LE, VISCII, WCHAR_T, WIN-SAMI-2, WINBALTRIM,
lot of things
But the command more on the file CreateTable.sql print :
CREATE TYPE enum_branchement_moderattachement AS ENUM(
$$Inconnu$$,
$$Relatif par rapport à l'amont électrique$$,
$$N'a jamais été rattaché$$,
$$Géographique$$,
$$Détaché$$,
$$Simplifié$$
);
What I did wrong ?
I solved my problem with those lines :
echo $LANG
that give me fr_FR
export LANG=fr_FR.utf8
Hope that can help someone else.
When i write '你' in agend and save it as test-unicode.txt in unicode mode,open it with xxd g:\\test-unicode.txt ,i got :
0000000: fffe 604f ..`O
1.fffe stand for little endian
2.the unicode of 你 is \x4f\x60
I want to write the 你 as 604f or 4f60 in the file.
output=open("g://test-unicode.txt","wb")
str1="你"
output.write(str1)
output.close()
error:
TypeError: 'str' does not support the buffer interface
When i change it into the following ,there is no errror.
output=open("g://test-unicode.txt","wb")
str1="你"
output.write(str1.encode())
output.close()
when open it with xxd g:\\test-unicode.txt ,i got :
0000000: e4bd a0 ...
How can i write 604f or 4f60 into my file the same way as microsoft aengda do(save as unicode format)?
"Unicode" as an encoding is actually UTF-16LE.
with open("g:/test-unicode.txt", "w", encoding="utf-16le") as output:
output.write(str1)
cpxSR2bnPUihaNxIFFA8Sc+8gUnWuJxJi8ywSW5ju0npWrFJHW2MSZAeMklcZ71IjrBySF2ci0gdecRI0vD/SM4ZF0m1ZSJJBY8bSZJl/0intaxIlQJBSPdY3EdBLM9Hp4wLSOK8Nki8L1pIoglxSAvNbkjHg0VIDlv7R6B2Y0elCqVGFWuVRgagAkdxHTdHELxRR9i2VkdyEUlHU84kRzTS2kalKFxG
This is a string from an XML file from my mass spectrometer. I am trying to write a program to load two such files, subtract one set of values from another, and write the results to a new file. According to the specification file for the .mzML format, the encoding of the numerical data is alleged to be base64. I can't convert this data string to anything legible using any of the many online base64 converter or using NotepaD++ and the MIME toolkit's base64 converter.
The string, in the context of the results file, looks like this:
<binaryDataArray encodedLength="224">
<cvParam cvRef="MS" accession="MS:1000515" name="intensity array" unitAccession="MS:1000131" unitName="number of counts" unitCvRef="MS"/>
<cvParam cvRef="MS" accession="MS:1000521" name="32-bit float" />
<cvParam cvRef="MS" accession="MS:1000576" name="no compression" />
<binary>cpxSR2bnPUihaNxIFFA8Sc+8gUnWuJxJi8ywSW5ju0npWrFJHW2MSZAeMklcZ71IjrBySF2ci0gdecRI0vD/SM4ZF0m1ZSJJBY8bSZJl/0intaxIlQJBSPdY3EdBLM9Hp4wLSOK8Nki8L1pIoglxSAvNbkjHg0VIDlv7R6B2Y0elCqVGFWuVRgagAkdxHTdHELxRR9i2VkdyEUlHU84kRzTS2kalKFxG</binary>
I can't proceed until I can work out what format this encoding is meant to be!
Thanks in advance for any replies.
You can use this trivial program to convert it to plaintext:
#include <stdio.h>
int main(void)
{
float f;
while (fread(&f, 1, 4, stdin) == 4)
printf("%f\n", f);
}
I compiled this to "floatdecode" and used this command:
echo "cpxSR2bnPUihaNxIFFA8Sc+8gUnWuJxJi8ywSW5ju0npWrFJHW2MSZAeMklcZ71IjrBySF2ci0gdecRI0vD/SM4ZF0m1ZSJJBY8bSZJl/0intaxIlQJBSPdY3EdBLM9Hp4wLSOK8Nki8L1pIoglxSAvNbkjHg0VIDlv7R6B2Y0elCqVGFWuVRgagAkdxHTdHELxRR9i2VkdyEUlHU84kRzTS2kalKFxG" | base64 -d | ./floatdecode
Output is:
53916.445312
194461.593750
451397.031250
771329.250000
1062809.875000
1283866.750000
1448337.375000
1535085.750000
1452893.125000
1150371.625000
729577.000000
387898.875000
248514.218750
285922.906250
402376.906250
524166.562500
618908.875000
665179.312500
637168.312500
523052.562500
353709.218750
197642.328125
112817.929688
106072.507812
142898.609375
187123.531250
223422.937500
246822.531250
244532.171875
202255.109375
128694.109375
58230.625000
21125.322266
19125.541016
33440.023438
46877.441406
53692.062500
54966.843750
51473.445312
42190.324219
28009.101562
14090.161133
Yet another Java Base64 decode with options to uncompress should you need it.
Vendor spec indicated "32-bit float" = IEEE-754 and specified little-endian.
Schmidt's converter shows the bit pattern for IEEE-754.
One more Notepad++ step to look at the hex codes:
Notepad++ TextFX plugin (after the Base64 decode you already did)
select the text
TextFX > TextFX Convert > Convert text to Hex-32
lets you look at the hex codes:
"000000000 72 9C 52 47 66 E7 3D 48- ... 6E 63 BB 49 |rœRGfç=H¡hÜHP
Little-endian: 47529C72 converts (via Schmidt) as shown above by David.
You can access such data from mzML files in Python through pymzML, a python interface to mzML files.
http://pymzml.github.com/