Python: test if a dictionary contains a specific key










def contains(dict, key):
return key in dict

dict={}
dict['a']=1
dict['b']=2
dict['c']=3

print(contains(dict, 'a')) # True
print(contains(dict, 'd')) # False



Python: delete a item from a list








  • remove the first matching element:
    if e in list:
    list.remove(e)
  • remove all occurrences:
    while e in list:
    list.remove(e)
  • EAFP style: remove the first occurrence:
    try:
    list.remove(e)
    except ValueError:
    # not in the list
    pass
  • EAFP style: remove all occurrences:

    while True:
    try:
    list.remove(e)
    except ValueError:
    break

EAFP

Easier to ask for forgiveness than permission. This common Python coding style assumes the existence of valid keys or attributes and catches exceptions if the assumption proves false. This clean and fast style is characterized by the presence of many try and except statements. The technique contrasts with the LBYL style common to many other languages such as C.




Python: del or assign None?









>>> x=1
>>> print x
1
>>> del x
>>> print x
Traceback (most recent call last):
File "", line 1, in
NameError: name 'x' is not defined

>>> x=1
>>> print x
1
>>> x=None
>>> print x
None

see also




Compile and install GNU-COBOL (formerly Open COBOL) on Mac OS X Yosemite








  1. Make sure you have xcode installed.
  2. Compile and install GNU MP Bignum Library.
  3. Compile and install Berkeley DB.
  4. Download GNU COBOL
  5. Compile and install:
    ./configure CFLAGS=-I/usr/local/include LDFLAGS=-L/usr/local/lib --prefix=/usr/local
    
    make
    
    sudo make install
      



Compile and install Berkeley DB on Mac OS X Yosemite








  • Make sure you have xcode installed.
  • Download Berkeley DB from Oracle.
  • Compile and install:
    unzip db-6.1.19.zip
    cd db-6.1.19/build_unix
    ../dist/configure --disable-replication --prefix=/usr/local
    make
    sudo make install
      
  • To check where the files are located, run
    find /usr/local | grep db
    you should see something like below:
    /usr/local/lib/libdb-6.1.a
    /usr/local/lib/libdb-6.1.dylib
    /usr/local/lib/libdb-6.1.la
    /usr/local/lib/libdb-6.dylib
    /usr/local/lib/libdb.a
    /usr/local/lib/libdb.dylib
    /usr/local/include/db.h
    /usr/local/include/db_cxx.h
    



Compile and install GNU MP Bignum Library on Mac OS X Yosemite








  • Make sure you have xcode installed.
  • Excute the following commands to download, compile and install gmp 6.0.0 into /usr/local.
    cd /tmp; wget https://gmplib.org/download/gmp/gmp-6.0.0a.tar.bz2; tar jxvf gmp-6.0.0a.tar.bz2; cd gmp-6.0.0
    
    ./configure --prefix=/usr/local
    
    make
    
    make check
    
    sudo make install
        
  • To check where the files are installed, run
    find /usr/local | grep gmp
    and you should see something like below:
    /usr/local/include/gmp.h
    /usr/local/include/gmpxx.h
    /usr/local/lib/libgmp.10.dylib
    /usr/local/lib/libgmp.a
    /usr/local/lib/libgmp.dylib
    /usr/local/lib/libgmp.la
    /usr/local/lib/libgmpxx.4.dylib
    /usr/local/lib/libgmpxx.a
    /usr/local/lib/libgmpxx.dylib
    /usr/local/lib/libgmpxx.la
    /usr/local/share/info/gmp.info
    /usr/local/share/info/gmp.info-1
    /usr/local/share/info/gmp.info-2

see also




synchronized method in python








Java code


class AClass {
private int count =0;

public synchronized inc(){
count++;
}

public synchronized dec(){
count--;
}
}

Python code


import threading

class AClass(object):
def __init__(self):
self.__count = 0;
self.__lock = threading.RLock()

def inc(self):
with self.__lock:
self.__count += 1

def dec(self):
with self.__lock:
self.__count -= 1


see also




synchronized static method in python








Java code


class AClass {
private static int _count = 0;

public static synchronized inc(){
_count ++;
}

public static synchronized dec(){
_count --;
}
}

Python code


import threading

class AClass(object):
__count = 0
__lock = threading.RLock()

@classmethod
def inc(cls):
with cls.__lock:
cls.__count += 1

@classmethod
def dec(cls):
with cls.__lock:
cls.__count -= 1



Use xargs to process files in parallel








find ./ -type f -iname '*.nii' | xargs -n 1 --max-procs 4 gzip



Generate index.html using tree command








  • To install:
    sudo apt-get install tree
  • To use:
    tree -H <base> -T <title> -o <output.html>
    for example:
    tree -H /maven -T "Mediaflux Maven Repository" -o index.html



X11 forwarding over ssh








ssh -XYC user@server xterm
The options are:
  1. X: enables x11 forwarding
  2. Y: enables trusted x11 forwarding
  3. C: compresses the communication
  4. xterm: can be substituted with other commands that are available on the server

X11 forwarding over multiple ssh hops

Suppose you do not have direct ssh access to server2. You can access server2 via server1 over ssh. (localhost => server1 => server2)
  1. In terminal 1:
    ssh server1 -L2200:server2:22
  2. In terminal 2:
    ssh localhost -XYC -p2200 xterm
    It will run xterm on server2 and displays back to localhost.



Free HDD Secure Erase Tools








  1. DBAN
  2. KillDisk
  3. Parted Magic