Summary of ways to create a list

See Lists may be constructed in several ways.

  1. With [square brackets].
    1. List the elements in square brackets, separated by commas. See List of numbers, List of strings. 
      months = [
    "January",  #0
    "February", #1
    "March"     #2
]
    2. Multiple copies of the same value. See the multiplication operator and often haunts. 
      myList = 5 * [0]

for i in myList:
    print(i)

      0
0
0
0
0
    3. Start with an empty list, and add additional items with append or insert. See CSV. 
      myList = []    #Start with an empty list.

for var in something:
    myList.append(anAdditionalItem)
    4. Use a list comprehension.
  2. Concatenate two existing lists. 
    bigList = [10, 20, 30] + [40, 50, 60]
bigList += [70, 80, 90]
print(bigList)

    [10, 20, 30, 40, 50, 60, 70, 80, 90]
  3. Split a string into a list of strings.
    1. Split a string into substrings using a separator string. See Split and splitlines. 
      preamble = """\
We hold these truths to be self-evident,
that all men are created equal,
that they are endowed by their Creator with certain unalienable Rights,
that among these are Life, Liberty, and the pursuit of Happiness.
"""

phrases = preamble.split(",\n") #phrases is a list of 4 strings.
    2. Split a string into individual characters with list. See Split. 
      fdr = "Franklin Delano Roosevelt"
characters = list(fdr)   #characters is a list of 25 strings, each of length 1
    3. Split one string into comma separated values. myString is a string containing comma separated values, and [myString] is a list containing one string. Since [myString] contains only one item, we call next only one time. myString could come from a text file; see CSV. 
      import csv

      reader = csv.reader([myString]) #[myString] is a list containing one string
fields = next(reader)           #fields is a list of strings.

  4. Read the lines of a text file into a list of strings.
    from disk from web
    Read the file
    into one big
    string of characters
    or one big sequence of bytes;
    then split it into lines
    using splitlines.    		 
    import sys

filename = "/Users/etc."

try:
    infile = open(filename)
except FileNotFoundError as error:
    print(error, file = sys.stderr)
    sys.exit(1)
except PermissionError as error:
    print(error, file = sys.stderr)
    sys.exit(1)

bigString = infile.read()      #bigString is one big string.
infile.close()

lines = bigString.splitlines() #lines is a list of strings
for line in lines:             #line is a string
    print(line)                #line does not end with a newline

    import sys
import urllib.request

url = "http://etc."

try:
    infile = urllib.request.urlopen(url)
except urllib.error.URLError as error:
    print(error, file = sys.stderr)
    sys.exit(1)

bigSequenceOfBytes = infile.read()
infile.close()

try:
    #Convert sequence of bytes to string of characters.
    bigStringOfCharacters = bigSequenceOfBytes.decode("utf-8")
except UnicodeError as error:
    print(error, file = sys.stderr)
    sys.exit(1)

#lines is a list of strings
lines = bigStringOfCharacters.splitlines()
for line in lines:
    print(line)             #line does not end with a newline
    Read the file
    into a list of lines
    using readlines.    		 
    import sys

filename = "/Users/etc."

try:
    infile = open(filename)
except FileNotFoundError as error:
    print(error, fie = sys.stderr)
    sys.exit(1)
except PermissionError as error:
    print(error, file = sys.stderr)
    sys.exit(1)

lines = infile.readlines() #lines is a list of strings
infile.close()

for line in lines:
    #end="" because each line already ends with a newline
    print(line, end = "")

    import sys
import urllib.request

url = "http://etc."

try:
    infile = urllib.request.urlopen(url)
except urllib.error.URLError as error:
    print(error, file = sys.stderr)
    sys.exit(1)

#lines is a list of sequences of bytes.
lines = infile.readlines()
infile.close()

try:
    #lines is now a list of strings of characters.
    lines = [line.decode("utf-8") for line in lines]
except UnicodeError as error:
    print(error, file = sys.stderr)
    sys.exit(1)

for line in lines:
    #end="" because each line already ends with a newline
    print(line, end = "")