Explaining a variable-passing problem a friend was having.

gistfile1.py

# Basics (things you already know)
"abc"                # the string "abc"
SomeVar = "abc"      # SomeVar is a variable whose value is the string "abc"
func()               # func is a function we're calling with no arguments
func("abc")          # call func and pass the string "abc" as the first argument
func(SomeVar)        # call func and pass the value stored in the variable SomeVar as the first argument
                     # the important thing here is that we pass an "expression" and we *evaluate*
                     # the expression and just pass the result of that...SomeVar *evaluates* to the string "abc"

# Extrapolating from the basics
"abc" + "def"        # append the string "def" to the string "abc". result is the string "abcdef" *string append may be different in Access
"def" + SomeVar      # append the value stored in SomeVar to the string "def". result is the string "defabc" if SomeVar contains "abc"
"def=SomeVar"        # the string "def=SomeVar" (SomeVar is not treated as a variable, since it's part of the string)

# Taking this to your Access problem:
Loc = DLookup("Location", "Part List", "PartNumber='abc'")

# DLookup is a function that takes three arguments...from looking at it I'd say it's the name
# of the field to return, the table name and a condition. All three are strings, and all three
# going to get dropped into a SQL query that looks something like this:
"SELECT Location FROM Part List WHERE PartNumber='abc'"
# the condition looks like [Field Name]=[value] and the value can either be another field name
# or a string. If it's a string, it has to be enclosed in single-quoted.  So when you try passing
# "PartNumber=ToolNum" to DLookup, you're actually passing that string, literally, as-is, rather
# than putting the value of the variable ToolNum into the string that you're passing. Then it
# creates SQL that looks something like this:
"... WHERE PartNumber=ToolNum"
# And so it's trying to look for the field named ToolNum to compare to the field named PartNumber.
# To do what you want you need to append the value stored in the variable ToolNum to the string
# "PartNumber=" before it gets passed into DLookup. Looks something like this:
Loc = DLookup("Location", "Part List", "PartNumber=" + ToolNum)

Strings, Variables and Function Calls

# Basics (things you already know)
"abc"                # the string "abc"
SomeVar = "abc"      # SomeVar is a variable whose value is the string "abc"
func()               # func is a function we're calling with no arguments
func("abc")          # call func and pass the string "abc" as the first argument
func(SomeVar)        # call func and pass the value stored in the variable SomeVar as the first argument

# Extrapolating from the basics
"abc" + "def"        # append the string "def" to the string "abc". result is the string "abcdef" *string append may be different in Access
"def" + SomeVar      # append the value stored in SomeVar to the string "def". result is the string "defabc" if SomeVar contains "abc"
"def=SomeVar"        # the string "def=SomeVar" (SomeVar is not treated as a variable, since it's part of the string)

# Taking this to your Access problem:
Loc = DLookup("Location", "Part List", "PartNumber='abc'")

# DLookup is a function that takes three arguments...from looking at it I'd say it's the name
# of the field to return, the table name and a condition. All three are strings, and all three
# going to get dropped into a SQL query that looks something like this:
"SELECT Location FROM Part List WHERE PartNumber='abc'"
# the condition looks like [Field Name]=[value] and the value can either be another field name
# or a string. If it's a string, it has to be enclosed in single-quoted.  So when you try passing
# "PartNumber=ToolNum" to DLookup, you're actually passing that string, literally, as-is, rather
# than putting the value of the variable ToolNum into the string that you're passing. Then it
# creates SQL that looks something like this:
"... WHERE PartNumber=ToolNum"
# And so it's trying to look for the field named ToolNum to compare to the field named PartNumber.
# To do what you want you need to append the value stored in the variable ToolNum to the string
# "PartNumber=" before it gets passed into DLookup. Looks something like this:
Loc = DLookup("Location", "Part List", "PartNumber=" + ToolNum)