Strings and Stuff in Python



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import numpy as np

from astropy import units as u

Strings are just arrays of characters



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s = 'spam'

s,len(s),s[0],s[0:2]



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s[::-1]

Arithmetic with Strings



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s = 'spam'
e = "eggs"

s + e



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s + " " + e



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4 * (s + " ") + e



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print(4 * (s + " ") + s + " and\n" + e)     # use \n to get a newline with the print function

You can compare strings



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"spam" == "good"



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"spam" != "good"



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"spam" == "spam"



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"sp" < "spam"



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"spam" < "eggs"

Python supports `Unicode` characters

You can enter unicode characters directly from the keyboard (depends on your operating system), or you can use the ASCII encoding.

A list of ASCII encoding can be found here.

For example the ASCII ecoding for the greek capital omega is U+03A9, so you can create the character with \U000003A9



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print("This resistor has a value of 100 k\U000003A9")



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Ω = 1e3

Ω + np.pi

Emoji are unicode characters, so you can use them a well (not all OSs will show all characters!)



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radio_active = "\U00002622"
wink = "\U0001F609"

print(radio_active + wink)

Emoji can not be used as variable names (at least not yet ...)



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☢ = 2.345

☢ ** 2

Watch out for variable types!



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n = 4

print("I would like " + n + " orders of spam")



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print("I would like " + str(n) + " orders of spam")

Use explicit formatting to avoid these errors

Python string formatting has the form:

{Variable Index: Format Type} .format(Variable)

Format Types d = Integer decimal g = Floating point format (Uses exponential format if exponent is less than -4) f = Floating point decimal x = hex s = String o = octal e = Floating point exponential b = binary



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A = 42
B = 1.23456
C = 1.23456e10
D = 'Forty Two'



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"I like the number {0:d}".format(A)



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"I like the number {0:s}".format(D)



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"The number {0:f} is fine, but not a cool as {1:d}".format(B,A)



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"The number {0:.3f} is fine, but not a cool as {1:d}".format(C,A)       # 3 places after decimal



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"The number {0:.3e} is fine, but not a cool as {1:d}".format(C,A)       # sci notation



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"{0:g} and {1:g} are the same format but different results".format(B,C)

Nice trick to convert number to a different base



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"Representation of the number {1:s} - dec: {0:d};  hex: {0:x};  oct: {0:o};  bin: {0:b}".format(A,D)

Formatting works with units



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NH_D = 34.47 * u.AU



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"The New Horizons spacecraft is {0:.1f} from the Sun".format(NH_D)



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"The New Horizons spacecraft is at a distance of {0.value:.1f} in the units of {0.unit:s} from the Sun".format(NH_D)

Formatting is way better than piecing strings together



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from astropy.table import QTable



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planet_table = QTable.read('Planets.csv', format='ascii.csv')



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for Idx,Val in enumerate(planet_table['Name']):
    
    a = planet_table['a'][Idx] * u.AU
    
    if (a < 3.0 * u.AU):
        Place = "Inner"
        
    else:
        Place = "Outer"
    
    S = "The planet {0:s}, at a distance of {1:.1f}, is in the {2:s} solar system".format(Val,a,Place)
   
    print(S)

Working with strings



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line = "My hovercraft is full of eels"

Find and Replace



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line.replace('eels', 'wheels')

Justification and Cleaning



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line.center(100)



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line.ljust(100)



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line.rjust(100, "*")



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line2 = "            My hovercraft is full of eels      "



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line2.strip()



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line3 = "*$*$*$*$*$*$*$*$My hovercraft is full of eels*$*$*$*$"



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line3.strip('*$')



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line3.lstrip('*$'), line3.rstrip('*$')

Splitting and Joining



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line.split()



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'_*_'.join(line.split())



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' '.join(line.split()[::-1])

Line Formatting



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anotherline = "mY hoVErCRaft iS fUlL oF eEELS"



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anotherline.upper()



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anotherline.lower()



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anotherline.title()



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anotherline.capitalize()



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anotherline.swapcase()

Regular Expression in Python (`re`)



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import re



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myline = "This is a test, this in only a test"



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print(myline)

Raw strings begin with a special prefix (r) and signal Python not to interpret backslashes and other special metacharacters in the string, allowing you to pass them through directly to the regular expression engine.



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regex1 = r"test"



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match1 = re.search(regex1, myline)



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match1



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myline[10:14]



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match3 = re.findall(regex1, myline)



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match3

One of the useful tings about regular expressions in Python is using the to search and replace parts of string (`re.sub`)



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mynewline = re.sub(regex1, "*TEST*", myline)



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mynewline

RegEx Golf!



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golf_file = open("golf_00").read().splitlines()



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golf_file



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for i in golf_file:
    print(i)



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def regex_test_list(mylist, myregex):
    
    for line in mylist:
        
        mytest = re.search(myregex, line)
        
        if (mytest):
            print(line + " YES")
        else:
            print(line + " NOPE")



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regex = r"one"



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regex_test_list(golf_file, regex)



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regex = r"t|n"



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regex_test_list(golf_file, regex)

Working with Files and Directories (OS agnostic)

The os package allows you to do operating system stuff without worrying about what system you are using



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import os



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os.chdir("./MyData")



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my_data_dir = os.listdir()



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my_data_dir



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for file in my_data_dir:
    
    if file.endswith(".txt"):
        print(file)



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for file in my_data_dir:
    
    if file.endswith(".txt"):
        print(os.path.abspath(file))

You can also find files with `glob`



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import glob



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my_files = glob.glob('02_*.fits')



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my_files



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for file in my_files:
    
    file_size = os.stat(file).st_size
    out_string = "The file {0} as a size of {1} bytes".format(file,file_size)
    
    print(out_string)



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