#set document(title: "14.2 Writing to files", author: "OpenStax / XYZ Homework") #set page(width: 8.5in, height: auto, margin: 1in) #import "@preview/cetz:0.5.2" #set text(font: ("STIX Two Text", "Libertinus Serif", "New Computer Modern"), size: 10.5pt, lang: "en") #show math.equation: set text(font: ("STIX Two Math", "New Computer Modern Math")) #set par(justify: true, leading: 0.62em, spacing: 0.9em) #set enum(spacing: 1.1em) // room between list items so tall inline fractions don't collide #set list(spacing: 1.1em) #set table(stroke: 0.5pt + rgb("#c7ccd3")) #let BLUE = rgb("#183B6F") // brand navy — section bars + example/solution labels (white on navy 11.09:1) #let ORANGE = rgb("#A94509") // brand primary-700 — AA-safe deep orange for TEXT (5.93:1 on white; raw brand #F37021 is 2.94:1 and must never carry text) #let RED = rgb("#DC2626") // brand error-600 #let GREEN = rgb("#059669") // brand success-600 (decoration only; small green text uses green-text #007942) #show heading.where(level: 1): it => block(width: 100%, above: 0pt, below: 16pt, fill: gradient.linear(BLUE, rgb("#2C5AA0")), inset: (x: 14pt, y: 12pt), radius: 3pt, text(fill: white, weight: "bold", size: 19pt, it.body)) #show heading.where(level: 2): it => block(width: 100%, above: 18pt, below: 10pt, fill: BLUE, inset: (x: 10pt, y: 6pt), radius: 2pt, text(fill: white, weight: "bold", size: 12pt, it.body)) #show heading.where(level: 3): it => text(fill: ORANGE, weight: "bold", size: 12.5pt, it.body) #show heading.where(level: 4): it => text(fill: BLUE, weight: "bold", size: 10.5pt, it.body) #let examplebox(label, title, body) = block(width: 100%, breakable: true, fill: rgb("#EFF1F5"), stroke: 0.5pt + rgb("#CFDDF0"), radius: 4pt, inset: 10pt, above: 12pt, below: 12pt)[ #block(below: 6pt)[#box(fill: BLUE, inset: (x: 6pt, y: 2pt), radius: 2pt, text(fill: white, weight: "bold", size: 8.5pt, label)) #h(0.4em) #strong[#title]] #body] // rail = decorative left rule (raw brand token); labelcolor = AA-safe label text shade #let notebox(label, rail, labelcolor, tint, body) = block(width: 100%, breakable: true, fill: tint, stroke: (left: 3pt + rail), inset: (left: 10pt, rest: 8pt), radius: (right: 4pt), above: 11pt, below: 11pt)[ #text(fill: labelcolor, weight: "bold", size: 7.5pt, tracking: 0.5pt)[#upper(label)] #linebreak() #body] #let solutionbox(body) = block(above: 4pt, below: 8pt)[ #text(fill: BLUE, weight: "bold", size: 8.5pt)[Solution] #linebreak() #body] #let figph(msg) = block(width: 100%, height: 60pt, fill: rgb("#f6f7f9"), stroke: (paint: rgb("#c7ccd3"), dash: "dashed"), radius: 4pt, inset: 10pt)[ #align(center + horizon, text(fill: rgb("#889"), style: "italic", size: 9pt, msg))] // Standardize inlined figure sizes: measure the natural CeTZ canvas, then scale to a // consistent envelope (aspect-aware; see build_typst.py FIG_* constants). Unlike the // print preamble, dimensions are FLOORED: in an editor a user can trim a figure to a // degenerate 1-D shape (a bare line), and w/h or tw/w would then divide by zero. #let _STD_W = 3.5 #let _WIDE_W = 5.6 #let _MAX_H = 3.4 #let _ASPECT_WIDE = 2.2 #let _UPSCALE_MAX = 1.15 #let stdfig(body) = context { let m = measure(body) let w = calc.max(m.width / 1in, 0.01) let h = calc.max(m.height / 1in, 0.01) let tw = if w / h > _ASPECT_WIDE { _WIDE_W } else { _STD_W } let s = calc.min(tw / w, _MAX_H / h, _UPSCALE_MAX) align(center, box(scale(x: s * 100%, y: s * 100%, reflow: true, body))) } #show figure: set block(breakable: false) #set figure(gap: 8pt) #show figure.caption: set text(size: 8.5pt, fill: rgb("#555")) == 14.2#h(0.6em)Writing to files === Learning objectives By the end of this section you should be able to - Understand how to open a file for writing. - Explain different modes for opening a file in Python. - Demonstrate the use of the write() function to write to a file. - Understand the importance of closing a file. === Opening a file for writing A file may be opened for reading, allowing no changes, or for writing, allowing changes to occur in the file. The #strong[mode] defines whether a file is opened for reading only or for writing. The default mode of the open() function is reading only. A second mode parameter defines the mode. Ex: open("output.txt", 'w') opens the #emph[output.txt] file in writing mode. The following table lists common modes. #figure(table( columns: 3, align: left, inset: 6pt, table.header([Parameter], [Mode name and description], [Example]), ['r'], [Read mode: #linebreak() Open the specified file for reading. #linebreak() If the file does not exist, an error occurs. #linebreak() When no mode parameter is used, the default is to open a file in read mode.], [fileobj = open("input.txt") #linebreak() same as: #linebreak() fileobj = open("input.txt", 'r')], ['w'], [Write mode: #linebreak() Open the specified file for writing. #linebreak() If the file does not exist, then the file is created. #linebreak() If the file already exists, the contents of the file are overwritten.], [fileobj = open("output.txt", 'w')], ['a'], [Append mode: #linebreak() Open the specified file for appending, which means adding information to the end of the existing file. #linebreak() If the file does not exist, then the file is created.], [fileobj = open("log.txt", 'a')], )) #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[File modes] Assume that a file named #emph[logfile.txt] exists. ] === Using write() and close() The #strong[write()] function is used to write to an already opened file. The write() function will only accept a string parameter. Other variable types must be cast to string before writing using write(). The write() function does not automatically add a newline character as the print() function does. A newline must be added explicitly by adding a newline ('\\n') character. The write() function writes automatically to a temporary store called the file buffer. To ensure that the information is written to a file, the close() function must be used. The close() function finalizes changes and closes the file. #examplebox("Example 1")[Writing and appending to a file][ """Operations for writing and appending to files.""" \# Create a new file opfile = open("output.txt", 'w') \# Writing to the file opfile.write("Writing to a new file.") \# To add another line the newline character must be used opfile.write("\\nSecond line.") \# Ensure changes are saved by closing the file opfile.close() \# Read and display the contents of the file infile = open("output.txt") print("\\nRead the original file:\\n") print(infile.read()) infile.close() \# Reopen the file in append mode to add to the file opfile = open("output.txt", 'a') \# Note the use of newline characters opfile.write("\\nAdding to the file.") opfile.write("\\nAdding another line.") \# Ensure changes are saved by closing the file opfile.close() \# Read and display the contents of the modified file infile = open("output.txt") print("\\nRead the modified file:\\n") print(infile.read()) infile.close() The code's output is: Read the original file: Writing to a new file. Second line. Read the modified file: Writing to a new file. Second line. Adding to the file. Adding another line. ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[write() and close()] #link("https://www.openstax.org/r/write-and-close")[write() and close(); ch 14, video 3] ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Writing to files] #emph[resources.txt] already exists. What would happen for each of the following snippets of code? ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Writing to a new file] Write a line of text to a file called #emph[out.txt]. Don't forget to use close(). #emph[out.txt] should only have the line of text that you have written. ] #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[ #emph[Writing to an existing file] Add two lines of text to the file called #emph[out.log], such that #emph[out.log] has three lines after the code executes. Hint: Don't forget that you need a newline character. ]